My mom has advanced dementia. We can no longer care for her at home. Any suggestions?

Martinjan, Please do not base any decisions on hoping for adjustment. It will likely not happen. You already are dealing with someone who has a type of dementia that causes anger and outbursts. That will be unlikely to change. What MAY change is the the facility will find it so difficult to deal with Mom that they will ask for neuro/psyc eval and some medications. Then you are between the Devil and the Deep Blue with worrying that Mom is too medicated versus Mom will lose her position because of lashing out at staff and or other patients.
There are in fact smaller in home nursing homes, run with often between 6 and 10 patients, sometimes with roommates and some times without. It is hard to evaluate any facility in Covid-19 times. But they will have a way.
The way I learned about the inhome places in a certain city was because I had contact with a man acting as a Financial Fiduciary. He was, by court order, in charge of several patients in my brother's facility. Acted as their guardian, their POA, their Health Care advocate. This was by appointment of the court mostly because of dysfunctional families. He was a marvel at knowing the availabiltiy of EVERYTHING. If you are in a decent sized area check out Fiduciaries with your search engine or ask MD or a Social Worker if they can recommend one. Make it clear you are buying an hour of his or her time for advice. Then take questions and specifically get a list of trusted providers in the area. Average cost for the fiduciary I know for an hour of time is 90.00 to give you an idea of So. California price. Better than 350.00 for Lawyer. Social Workers may know of places. And then just google care homes in your area. Good luck.

Martinjan, moving someone during covid may be both good news/bad news timing: the bad news is if she goes into LTC or MC she is where a lot of the virus activity has had an impact, and your visits will be very restricted; the good news is your visits will be very restricted as she "adjusts". Please have no expectations for "settling in" or adjusting. I think this is where much of the distress of placing an LO comes from, that we assumed that eventually their attitude would get better and they'd be engaged in activities, etc. By your own description your mom is already not that person. It may take a long while, a very long while, for her to "settle" in. She can only do what her dementia allows her to do. Try not to romanticize it. When we transitioned my MIL into AL due to short-term memory loss, we had all these images of her chatting with people, playing cards, going on field trips...after all she was in a really nice place. Nope. None of that happened. Eventually she refused to get out of bed, even though she could. Today, instead of being semi-independent in a cute little apartment in AL she is languishing in bed, not even getting out to eat meals with others. She was a social person prior to all of her decline. I'm not trying to depress you, but Americans see a lot of romanticization of senior living that mostly isn't reality. Just suggesting you have no expectations of what happens once she's in there. We on this forum understand your journey. I wish you peace in your heart and as successful a transition as possible.

With advanced Dementia your POA can kick in. Read ur POA to see what, if anything, is needed to put it in effect. If a doctor's note is needed then get it. You may want him to also declare her needing 24/7 care.
Once in effect, it doesn't matter what Mom wants its what she (and you) needs. I would not even discuss it with her. Find a nice AL or if she can't afford that, apply for Medicaid but that means LTC. When I placed Mom, we told her she was going to a nice apartment the day we moved her.