Well, you don't necessarily have to get them to like you to work for you at the moment. You can ask if they will serve you and they usually have a "buy-out price" based on how much they like you. It's basically the same as buying a slave. They work for you no questions but still hate you. Slaves are way cheaper, so unless you REALLY like someone you captured, sell them. I do think a fear/spirit-type system should go in place. Currently, the "Warden" trait only gives about 1-2 relationship a day if you have someone with the trait assigned to a jail cell (not a binding cell sadly) with a prisoner in it. So I think a "Torturer" trait would be a good alternative to that after a "dungeon" or "torture cell" is added. To note, anyone assigned to a jail cell will feed and clean the prisoners. A setting to tell the torturer whether you want them to clean and feed certain prisoners should be added, I ain't about to assume DM wants their slaves treated nicely (or certain ones). I know I've had a couple invaders that have been complete jerks.
Yes, forcing them to work the moment they are captured should also be a thing IMO. But I'd wager you would need to assign a guard to the room they are assigned to. Since removing their gear doesn't happen in this game, if they get free they'll just run rampent again.