Sorry to be harsh but your reducing your hours/ quitting your job is about as helpfull as tit on a bull. All that will do is push the emotional and financial hardship into your family too.
And what @DeathStare said.
You are thinking about going through with this and cant work out how you will cope. Your Mum is looking at her life shrinking down into the role of caregiver with no plan of action or timeline of how things will change or end in sight.
You are "popping" in to your Dads home for short periods of time and if he likes you he will be "switched on" to keep you happy, the friendly loveable toddler who is all smiles and wants to be friends.
On the other hand your Mum also has the stroppy toddler, a full strenght adult who can and will say no. Who is more and more focused on his wants and needs with an ever reducing ability to even recognise that your Mum needs emotional support.
Have you realised that she was having a breakdown because you did not turn up to give her that couple of hours of breathing room? You are thinking of leaving your career to help him how trapped by love and obligation do you think she feels?
Why should your Mum be expected to be a full time 24/7 caregiver and spend 100% of the joint assets on your Dad? Is that fair? What is the plan for her "Old Age" care?
The very best thing you can do is ask your Mum what she wants to have happen in the future. What help would she need to keep your Dad at home or should the discussion be about carehomes and funding plus the division of assets.
If and only if your Mum is still ok with being his full time carer should you begin to look at what support is needed to manage your Dads condition.
The starting point of your assessment is that your Mum is not a carer but rather he lives alone. What is he able to do?
What is he like at night is he sleeping through or up and down for toileting or just active.
What is he like during the day? Can he actually get up wash and dress himself or feed and hydrate or take his meds without prompting etc. Is he actually safe to be left alone for periods of time? Is he functional going out?
You should contact one if the organisations which offer support for his condition to see what they day-to-day advice they can recommend.
If your Mum is getting falling down drunk to unwind or to hide from her life or she is an alcoholic who cant cope she needs professional help to put a plan in place which works for both her and your Dad so you should call adult social services for an assessment.
You may note that I had not suggested you have a discussion with your Dad. You need to seek help for your Mum first.
And on the dememtia issue.
If they are moving to down size is the place they are moving to set up to allow for a wheelchair/hoists and two caregivers providing in home care?
Can your Dad actually cope with the massive change of a new house new location?
What if any legal protections have been put in place to allow your Mum access and manage his "share" of the assets and to protect her share? What happens if she were to die or be incapacitated heeself?
Etc.
Read some if the threads :
https://www.mumsnet.com/talk/elderly_parents
https://www.mumsnet.com/talk/dementia
to get an idea if what may be ahead for both of you.