This is such an inspiring manga! It has kept my imagination active for a long time thinking of various ways it could have been continued/changed. I've thought about stuff like: the boys forming a cult around the ass, them keeping record of her appearance frequency and time then being rougher and/or forcing her to stay around longer if she has been late or skipping, inserting some gifts to her ass, adding unremovable buttplug so that she'll have to come around to shit, my latest thought is that the boys could have a vacation coming up they should somehow figure out a way to make sure the ass gets satisfied even when they aren't there leading to them building and programming a machine that does everything necessary.
This is super awesome but based on the princess complaining about broken promise later on I think in page 8 the witch meant she'll release her after 10 days or even earlier if she finds someone instead of sort of saying that both conditions must be met.