I was in a similar situation to you in 2018 when my dad had to move out of the family home. At that point we didn't know there was much wrong, although he said to me his brain wasn't working properly, I told him to go to the Dr if he was worried. I knew he was stressed about the move and would be trying to think of any excuse to not move but the house had been sold. At the time my younger brother was living with him so I took his view on it and he said dad was being difficult but wasn't unwell.
Dad was able to organise a move into assisted living, albeit not very well - we have lost all our family photos, items of sentimental value and almost all furniture as he only got one van to move and took mostly rubbish. We agreed that assisted living would be helpful as we felt it was too early for full time care. However I then started getting phone calls - dad tried to go back to the house, dad didn't pay the removal man, dad hadn't paid his rent, dad was wandering, dad has lost his car, dad can't use his mobile phone, dad has been banned from the canteen.
We went to see dad at Christmas and he was living out of bin bags with newspaper spread over the bed and in the bathroom on the floor, and the place was filthy. Not a total surprise as dad has always lived in chaos and never cleaned anything. He'd forgotten we were coming. My poor nephew was shocked. Social services assured me he was fine. I disagreed.
I sent in a care agency and the woman from it called me, horrified at dad's state of living and the fact he'd not eaten that day due to the canteen issue.
Shortly after that he was served an eviction notice by the assisted living facility and with no diagnosis he was taken into respite to do some tests.
I later found him a self funded place at the same home and moved him in. At the time dad was still going out and it felt like using a sledgehammer to crack a nut but dad of course has only got worse and only a little over a year later he has been assessed as needing the highest level of care by the local authority and diagnosed with mixed dementia.
Dad used to go out into town and to the shop and so on but now he's fully in his own world and shows little interest in going out except for when frantic with delusions about his car, which was crushed long ago.
That was long, sorry, but my experience is that it's hard to know how bad it truly is and in general it's easier to get into the home early, even earlier than you think is necessary and have the decline supervised, especially if like me you aren't close by. You can come and go at homes and they should aim to maintain independence as long as possible.