Unfortunately, dh and I had to deal with Alzheimers on both sides over the past 5 years. My best friend is dealing with it in two of her relatives now. I would revisit the idea of keeping your mother home permanently. My ILs had always talked about staying at home no matter what, from the time I met them many decades earlier. They viewed assisted living as a horrible, horrible situation to be avoided at all costs.
My FIL had mild to moderate Alzheimers and suffered a fall that killed him. Up until then, my MIL was caring for him, it was doable at the time, but probably would not have been if he'd lived much longer. It was very hard on her, but she was also resistant to getting in anyone to help with him. We did as much as we could, but both work and had school aged kids at the time.
After he died, MIL lived in the family home for a few years until it became increasingly obvious that she too was getting dementia and it became unsafe for her to be alone. She was also resistant to getting anyone in to help. After a few incidents, we had to have a bit of an intervention. My BIL wanted her to move in with him. She did and it was not a happy situation. My SIL was saddled with taking care of her full time, while BIL largely ignored her. MIL deteriorated quite a bit, was very unhappy, although she still was terrified of the idea of moving to a care facility. It was a lonely, socially isolated situation for MIL even though she was living with them. She cried a lot and was very disoriented and demanding. SIL finally had enough.
I researched the memory care facilities in the area very thoroughly. There are some horrifyingly bad ones. There are also some surprisingly good ones. We were able to get MIL into my top choice. Although she wasn't happy about moving initially, she absolutely blossomed once she settled in. The social aspect of it was tremendously good for her. We visited several times a week, and she usually was surrounded with other ladies her age, and they'd all tell each other the same stories over and over again, but it didn't matter cause nobody remembered anything. She was happy again. My one regret is that we let BIL bully everyone into having MIL move in with him instead of moving her to the memory care place earlier.
The staff at this place is amazing. They have all sorts of little tricks to deal with the residents and make their lives happier. My older daughter ended up working with their activities department during her college vacations. Every day there is programming, a little exercise, outings for the more competent residents, time spent outdoors. It's a surprisingly cheerful, homey place.
It wasn't cheap. However, if you have caregivers in frequently, that's pretty expensive, too. My friend did that for a year or so for her mother. It's also a ton of work in scheduling and keeping things covered. People with Alzheimers can be really difficult to the caregivers, and there can be a lot of turnover in a low paid, thankless position.
I guess what I'm saying is be flexible and realize that home really may not be the best place for your mother when things start to decline. This is also going to be really hard on your dad. Be sure that you can check in often because in my experience, the spouse who is caring for the patient will try to minimize how bad the situation is because any kind of change is scary.
Your parents are fortunate in that they have ample assets. If your mom really does have Alzheimers, she isn't likely to live with it for 20 years. Talk to her doctor. While no one knows for sure, my BIL (who is a physician) told us that MIL was unlikely to live more than 5 years after diagnosis, and that ended up being true, although she was otherwise pretty healthy.
I'm sorry you are dealing with this, it's a dreadful disease.