Belly-riding sounds wonderful! Very hot idea, and I've always liked the idea of facing up doing it, giving adoring kisses underneath while being filled.
For not-directly-sexual parts, I think riding the 'regular' way could be very cute! And still interpretable as fitting the dom/sub direction, too. Something like... you're small and cute and far less capable for traveling, so you need to just hold on and let her take you where she wants you to go. (I'm not sure if I quite got it into words how I'd like, but I hope that communicates some of it.)
One specific thing, I have to say the setup of having to sacrifice multiple bits of unique equipment of the kind most RPG players store and hoard forever as part of submission is brilliant. Very clever integration of game systems with the dynamic.
And for one of the other bits of feedback... I'm really enjoying about, 95-98% of this. Like, a lot! Dominant centaur women are something I'm very much here for. Unfortunately for me, the word 'mommy' in a sexual context is a significant squick* for me, it really tends to put me off; it says something about the rest that I'm trying to read around it, I think. If it was possible to do anything there (and I could understand if not), I'm not really sure if it should be an in-universe sort of question (maybe if the word was one you brought up first, to retain the feeling of not actually saying no to anything she's doing...).
*Is this still a term people use? Sort of an older fandom word, meant as a 'this thing is really not for me' statement in an entirely non-judgmental way.