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

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

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.

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.

Firstly, I'd say it may be impossible because deep down you're not one of those people who can half-arse it. That's not a criticism of people who can btw, we're all different. You may be going above and beyond as that's who you are.

I haven't finished it yet, but a couple of colleagues have recommended the book "Four Thousand Weeks" which is a book around productivity and the mindset around it. The key point it's made so far is one that sounds backwards at first - don't look for ways to finish tasks easier/quicker (like a PP has suggested), that just gives you more time to do more tasks. It's about finding the balance that you seem to want.

Don’t reply to emails unless they are address to you or you are given a specific action. Lots of people send broadcast emails to a group or people maybe looking for help or information. The minute you answer you become the solution. Saves me loads of time as I had a habit of being a fixer.

If you are going the "quiet quitting" route then you need to have some viable change in sight. Such as retirement, another role or some such. If you have always worked hard and conscientiously its difficult to slow down without it being noticed and acted upon.

I did this in the year before I knew i was leaving to go to uni. No AI then to help out but plenty of little tricks. Like always having an answer ready as to why XYZ was not done and why the work had piled up. Always looking busy in work time. The simple answer was that things I used to do in my own time were now being done in work time. I was using my private time for the year long process of researching unis and courses. There was no internet then. You went to the library and you ploughed through prospectuses.

I had an appraisal with my boss who told me that he felt I was "not being quite so productive" as in the past. I agreed with him and informed him that this would be the last appraisal interview we would ever have. He just assumed I was looking for another job but i refused to discuss my personal affairs.

I resigned six weeks later.

I completely understand this. Hard workers can get taken advantage of. Do your best in the time you have but stop going over and above.
Start monitoring your hours and logging off at exactly your finish time. Take your full allocated break. If something is complex then allow extra time. Don’t volunteer for extra tasks. Don’t volunteer for anything. If extra work is allocated day you don’t have capacity.
It’s hard to put these boundaries down if you haven’t had them previously had them in place so consider applying for a role elsewhere.

The unhappiest people where I work are those who try least. For my own self worth I work hard.

Rather than trying to fuck around more, can you find a job where the satisfaction is inherent? Charity, government, work with some point to it.

What does your line manager say about your performance? I work with a couple of people who do ‘too much’. They are great performers but when resentment starts to build in it can be counterproductive and lead to burn out and team issues.
Start with timing. Keep to your contract hours. Then plan in enough ‘down time’ to keep on top of admin and any tasks that stress you. Some people book in space in their calendar for this to avoid meetings taking over.
Do you feel fairly remunerated for what you do? Can you discuss pay against performance so you at least don’t feel taken advantage of? (I’m in the Civil Service and it’s very hard to match pay to performance!).

The unhappiest people where I work are those who try least. For my own self worth I work hard.

Rather than trying to fuck around more, can you find a job where the satisfaction is inherent? Charity, government, work with some point to it.