Is your SO not bothered by the relative's living habits??

Not really. They're used to messiness and find it comforting. And to them the relative's room is "clean," because to them even the hoarder house is not that bad. Their only concept of "dirty" is when other people get upset by the mess, but they themselves have no intuitive concept of things being dirty. The only reason why they keep their own possessions relatively maintained is because I wouldn't allow it to get out of hand.

Since the relative has funds to pay for household expenses... would moving to an assisted living facility be an option?

It would be financially, but SO would see it as abandoning relative and relative would see it as abandonment also. I mean, relative sees it as abandonment if we don't eat food that they cooked, or in the past when we didn't want to sleep over in the hoarder house, so this would definitely be considered to be an unforgivable offense.

It's good that you have insight into how hoarding may affect others. Unfortunately I think relative has zero insight into that they are a hoarder and it is unlikely they will ever have insight. SO understands intellectually that relative is a hoarder, but not intuitively. And SO still gets upset and sees it as "insulting" if I refer to the house as a hoard or relative as a hoarder.

Thanks.