I see it from both sides - I have a job that can be all consuming too and I also have an elderly dad who has lots of health problems, not to mention really struggling with the day to day admin of his home and life. I do as much as I can but even though I’m not a full time carer, at times it’s completely overwhelming trying to juggle it all.
I remember one time coming home after my dad had been really sick for 4 weeks and I cared for him non stop - I raced straight to the fridge and got mashed on a bottle of wine. I’m not normally a massive drinker but I just needed to ‘escape’ and feel something other than constant worry and stress.
I think full time carers deserve a medal - they don’t get any ‘pay’ (Carers Allowance, if they’re entitled to it, doesn’t count) and they carry the whole load - everything from the emotional worry through to toileting. It can be unrelenting. The country is often held together by unpaid carers like your mum, who do a valuable service in keeping people out of the care/social services system.
Even if your mum is a difficult character, you can have a chat and say ‘how can I support you and dad?’ I would advise (as others have said) against cutting your hours but instead, if there’s money, outsourcing help. Get them a cleaner or someone to sit with dad so mum can go out or…whatever she needs to have a regular break. I’ve got a good set-up for my dad now. He has carers several times a day, there’s a cleaner plus a lady who collects and returns the washing. He’s also got a good local community of friends and that combined with what I do, we muddle through. If there isn’t much money, they’ll still be able to access help from social services, whether that’s little adjustments round the house or care packages/befriending services.
If you haven’t already, do the legal stuff and have honest conversations about what you’ll do when this progresses. Lots of people with dementia live very successfully at home but their carers need a lot of support - it’s too much for one person to carry alone.