In my opinion, this has nothing to do with what his machine could render. And it's not for the money also, he has paused the billing quite some time now. So it's probably more a hobby/side work for him.
Also he's a kind of perfectionist when it comes to the 3D scenery/assets. Havn't really seen one used in another game, so he is doing them mostly from scratch, too. Making that is for sure very time consuming also. But it seems that's the way he want's to do the game. For me this details in his renders are the big difference to most of the other games from devs doing that not as a profession.
And I also think there was a "little burnout" from the last releases, so he decided to "slow down" even a bit more and took away the pressure of Patrons with stopping the billings. And hiring others would get him definitely even into more stress, as said it's not easy to find someone you can completely trust your work. And I also think he WANTS to do the work himself, not someone else doing it.
Well, we can only wait and hope for the next release. And according to his postings, he still likes doing that. Just not as fast as others, and therefore we have to be a little more patient here.