Quick update, but even before the neutering (still four days away) his behavior has already been improving, because I know when to not interact with him. He's started taking treats from me (before I would try to offer them, he would ignore me at best or try to physically move my hand away at worst, so I would leave them and then he would eat them), he's started getting excited to see me (perks up and runs over to his cage door), and has started actually coming up to me and asking for pets (I think), and he lets me pet him while he's eating now (before he would just stop and wait for me to leave him alone, then resume eating).

He is still cage-bound, but it's on a table next to me while I work (a foot away) and I give him exercise time. He's started marking a pillow I gave to him when he was allowed to roam freely (so I don't really mind), so that's still behavior for him, but generally he is a lot less aggressive towards me now. I'm hoping to have him return to being a house-pet shortly after his neutering, and I'm considering trying to build a way for him to get up on the table so his home can permanently be right next to me.