Imo your suggestion makes sense. I think it would be still too easy, though: once you buy the alchemy room and unlock the right spells, you can make a slave obedient the same day you have captured her (or the day after, if you just rape her).
I would add an additional constraint: she must have lived with you a whole week at least (metrics.ownership >= 7 . Or metrics.ownership - metrics.jail >= 7 if you do not count the days spent in prison).