Hi OP. Sorry about your DH
You have basically described my FIL. He is a hoarder and I suspect on the autistic spectrum.
He lives in isolation and is isolating, even when his wife was alive - she died in 2020. He is 79.
I find him extremely hard work due to his negativity and his inability to see how bad his house is - I have known him nearly 20 years. He fixates on sorting out a cutlery drawer but doesn't see the room you can hardly get into due to all the shit in there. We have offered to help but it is all 'Fine'
Your DH WILL be affecting your DC.
DH has 2 DB's. All of them are in their 40s.
DH said when he went to secondary school he stopped inviting his friends around as he was so embarrassed by his home (I found this heart breaking)
He left for uni and never really went back.
As adults they rarely go back to the 'family' home as it is just so awful to be in. We go to my mum and dad's more and they live 4 hours away, FIL lives an hour away.
If your DC get partners, they might find your environment very difficult to be in and avoid coming to visit. I would never stop my DH from visiting his dad but I don't encourage it either. I actively try to minimise the amount of time my DC have with my FIL as he is so negative all the time, fixates on weird stuff and I don't want them to think that his house is normal.
1 of DH's DB is a carbon copy of his DF. No friends, no interests, very negative. I can't see him forming any sort of relationship whether it be friendship or something deeper.
The other DB has married a lovely woman, my SIL, but she is a hoarder and he doesn't know how to stop this vicious cycle so he enables her. I guess it's what he knows.
All the brothers are introverts and find it harder to very difficult to make friends. My DH is the most 'extrovert' and independent of all of them, although if you met him you'd probably think he's a quiet man.
They describe their childhood Christmases as dull and boring.
They could have had such different outcomes if they had got away from FIL. MIL was young and naive and essentially enabled his lifestyle.
FIL has not and will not ever change.
You need to decide whether you can put up with this (you have for 18 years!) or whether this is genuinely good for your DC. Personally I would go and see a divorce lawyer, the house being entirely his doesn't sound right.