How to quietly reduce my work ethic and rate to keep ticking over rather than going above and beyond at work.

If I didn’t the work would be an issue and perhaps performance could be implicated. How does this work of there is too much work humanly possible to do in the time allowed?

It works because you have the evidence to back it up. If performance is implicated you can show that just doing your own work is no problem performance wise but you have been asked to take on additional work and cover absences, etc. which accounts for some work not getting done. Just pass it back to them.

I used to be like you - always picking up stuff that nobody else would.
Then I stopped.
I sat on several committees. So I went to the chair of each committee and said I was stepping back. Each one agreed I'd made an invaluable contribution and agreed it was time the baton was passed to someone else. Result.
I managed a number of small budgets. I approached the relevant managers and explained that this was a legacy arrangement, that my role (and that of other staff) had evolved, and that it was no longer appropriate me to manage the particular budget. Again, I was thanked, and each one agreed with me. Result.
Since the impact on each manager was minimal, nobody joined the dots, and I was able to wind down to retirement without attracting attention.

Otherwise, all you can do is repeatedly go to your manager to point out that you can't do everything that's landing on your plate in your contracted hours, and ask them to prioritise. Every day, if necessary.

You could AI the shit out of that.

Which part of your role takes you the longest to complete?

If you're stressed, I would speak to your employer and make notes of your conversation. If they don't make changes go off sick for a few days. Go back, if nothing changes, go off sick again. Surely the benefit of being permanent is that they have a duty of care to you.

Stop working over and take your lunchbreaks.

If need be inform your manager that you are doing this.

If they need you to work more hours then they can pay you overtime.

Definitely play about with ChatGPT. Maybe create a template using it and then just ask it to use it using the new information you type in or even better still, upload the file it can extract the data from.

It can automate so much of your work, you just need to figure out what you need it to do. It’ll be done in seconds/minutes whatever it is.

I understand this. I was once asked to do something because someone else on the same grade as me wouldn't. This was said with a smile like some pseudo compliment. It riled me for ages. Definitely stick to your contracted hours. Stop volunteering- it's hard i know.

I would speak with your manager about your level of stress at work and it’s impacting your health, how the additional workload covering XYZ is too much and the deadlines can no longer be met.

Push back to him to prioritise repeatedly. At lunch time so some of your own stuff - finances/on-line shopping if being away from your desk is frowned upon.

Stop staying late.

Your manager needs to realise you will be off sick with stress if you don’t have a lunch break an start sticking to your hours. Can you do some exercise classes after work so you need to leave on time?

Good advice.
I started going for a walk at lunchtimes - it got me fresh air, a chance to let my brain think about other things, it got me some much-needed exercise, and people got used to me not being permanently available. There was no negative impact on anyone.
A colleague went as far as getting an informal agreement with his manager to come in about half an hour after everyone else so that he could go to the gym. Every morning! Now, the gym is not for me, but I silently applauded his approach, and wished I'd thought of that, instead of arriving early to beat the traffic (and to get a head start before the majority of my colleagues arrived - idiot!).

It's so easy to let work build up, incrementally. And if you find it hard to say no (like I was), then one day you realise you are overwhelmed. I recall one night when I couldn't sleep, getting up and making a long To Do list. Against each task, I marked who else could take it on. It was only a few, which was worrying, so I decided I had to act before I sank under the impossible burden.

I retired before ChatGPT etc was a thing, but I'm sure it's worth looking into, even if it only helps with part of your workload. For the rest, you'll need to be more pro-active - time to talk to your manager.

You sound like you hate your job… why not job hunt?! How long have you been there?

Other people’s ‘reasonable’ adjustments are not reasonable if they are impacting on you. They should be granted in context not in isolation. This is lazy HR-ing

are you in a Union?

Look at what the ACAS website says about reasonable adjustments

Out of all your tasks, identify the ones that have to be done versus nice to be done. Then identify the ones that have a highly visible impact. Focus on those so that everyone can see you putting in effort. This allows you to put in less effort elsewhere.

Obviously this depends on the exact nature of your work, but I am sure there are some things you can drop.

The first sentence is so wise.

But as someone who has worked in Local government all my life I would not suggest LG for a satisfying work life. Though it does depend in the team you work in and management culture. Its just that LG is dreadful at managing managers so there are many, many shit ones who do not leave until retirement.

Structure your day in a way that makes sense for you, and take time for other parts of career development which more directly benefit you.
For example, take the morning coffee time to draft your to do list and ease into work. Take the lunch break with a colleague or mentor, or take the extra 20 mins to walk around the block.
If a normal amount of work is producing 5 widgets per day, then produce your five widgets. If there are not enough hours in the day then that’s an issue for management as to how widgets are divided up or maybe more widget producers are needed.

Id also spend some time figuring out what your motivation is. If they paid you an extra 20 percent would you produce the 20 percent more widgets happily? Or do you not get satisfaction from the job? Is there a way to get that eg doing volunteer work or managing juniors, training, or whatever?

Do you get paid overtime?

I'm guessing you probably don't. So sit down and calculate your hourly salary rate for your set hours, then do it again for the hours you actually work. I did that once and it was a big shock to the system. After that, I started leaving on time. Just downed tools and left at 5pm. Caused a bit of a shock to management. How dare I leave on time? They got used to it though.

Getting the work done is your line manager's reponsibility, not yours. Just because they have delegated the work to you does not mean that they have also delegated the ultimate responsibility for it. They get paid more than you do and they aren't delegating any of their salary, are they?

I recently told my manager I was struggling with work life balance (I work part-time and was working on my days off) and talked through strategies to manage this. Now she tells me off (lightheartedly) if she hears from me on a non-working day.
If you set having better work life balance as a goal with your line manager, you can use this as a reason that things haven't got done. ('No, I've not looked at that yet. I prioritised this other thing for this reason and also got something else done. Do you agree with those the right priorities? I also managed a 20 minute lunch break and only stayed 5 minutes over yesterday so my targets are going well!')

This was me, with a huge side helping of imposter syndrome, I didn’t even realise I was doing 2 people’s work, I thought I must be inefficient and kept trying harder. I got really really ill in the end, got a medical retirement. The first thing is Don’t Tell Anyone that you are doing it. Set yourself some targets for the client facing stuff, never let it overrun, work towards finishing 5 or 10 minutes early and then staying put while you complete any admin. Wear headphones whenever you can, people will look for someone else to interrupt. If they still come to you, just say you are in flow and could they send you an email. Do emails twice a day, first thing and last thing. Don’t check in between.
Care less, everyone else may be equally busy, or have reasonable adjustments, but you are not their pack animal, your responsibility is to yourself and your family, work don’t care about you or they wouldn’t have put you in this situation. Keep your work throughput as it is, get it done quicker, but hang on to it , waiting for further info or needing to think about it a little longer. Take a walk every lunchtime. Start with 10 minutes, add 5 minutes each week. 45 minutes walking,preferably in green spaces , kills stress. It’s actually very productive- explain that it helps you work through stuff, because it will.
Stop killing yourself to schedule medical appointments and family stuff out of office hours. No one else does, if you think that’s not your workplace culture then they are doing it discreetly, and so can you. Don’t accept additional responsibilities without asking for adjustments to your workload, really, everytime. Never sack off training, see it as a little r&r . Start working towards only doing your paid hours, leave a little earlier , again increasing incrementally.
You will make yourself some time , if you can do this. Please don’t immediately put it into your domestic situation, start some new habits that are for you, a swim on the way home, a drink with a friend, date night or whatever. Look after yourself.

mantra seems to be ‘suck it up

That won't stand up in any documented correspondence. You need to start a paper trail. Inform your manager, in writing, of the additional tasks that you are unable to meet within your work time. And take your lunch break!