A few off the top of my head that may or may not fit, saw a few in this thread that I thought were great. Tbh I didn't really see a good thread for just general slave stuff so will put a few of those in too.
--- Games with Slavery, either in story or as a bad end ---
Slave Sword 1/2 - Sort of a bit lighter on the interesting aspects of slavery but kind of fits the bill
King's Exit - People mentioned Demon's Roots, this game came before that one. Translation is a bit spotty but has similar themed ends as DR, even if it doesn't have a "route".
Ninja Rape Legend Ajisai - Bad end(s) involving slavery/being trained & sold
Wings of Roldea - Has public use/training elements
Fiendish Violation Quest - Don't remember if this one fits into this category or the 2nd one but it has lengthy enslavement scenes/bad ends.
One Girl's Adventure in Another World - Training/Auction Bad End
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- This one is also kind of in between imo, entire game is basically about being forced to do shit because of debt but has some moderate length bad ends including at least one for slavery
--- More lengthy slave gameplay ---
Watashi, Kawaretemasu - Likely would need a translator like mtool for this one but this entire game is about the MC being trained. May have to look around a bit to find it and it's a VN but tbh I felt like this hits the mark the most.
Erina's Slave Diary - Don't remember this one too much but pretty sure you're a slave the whole game
Fallen Valkyrie - None currently but next patch is going to add something like this, game is similar to Cursed Armor which is highly recommended. No real definite timeline on when that patch is coming though.
Belial Red - Get enslaved pretty early in this game, can go into service routes with heavy H focus
Bad End Slave - Just a slave the entire game, as title suggests had only a bad end
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There are a good amount more I could add but a few of the better choices are covered here or in other threads. Some I can think of either fit less into what I think the general theme you're going for is or they're from the male perspective (like Tsurumiku VNs). Some also just can't be posted here, which is totally understandable and frankly in many regards commendable.