I don't know if this helps or not - another user was fumbling through Mainstage + MODX and didn't detail, in the forum message, everything that was done to resolve their issue(s). However, there are videos that may cover those topics (I didn't take a look at them yet). Maybe this will help:

https://www.yamahasynth.com/forum/modx-with-mainstage-audio-and-program-changes

This one has a link to Mainstage information and another user's additional information:

https://www.yamahasynth.com/forum/modx7-controller-and-mainstage

Not too terribly detailed - but some pointers from a user who integrated MODX+Mainstage (off-forum):

https://forums.musicplayer.com/topic/178186-dumb-midi-mainstage-question/#comment-2834631

I don't have Mainstage (or Logic) so I can't really dig in directly. Unfortunately - which limits what I can do.

However, given MODX's USB connection is a built-in audio interface (both directions) - it seems that Mainstage's sounds and MODX's can all be handled without using another audio interface. I use an external audio interface when I run out of inputs to go through "MODX" (Montage for me). In my case I have several keyboards and connect these to a multi-channel audio interface which gives more flexibility in the amount of devices I can record without having to re-cable everything. Although the re-cable thing is really just me being lazy (admittedly). My Montage is in and out of the studio for gigs/practice/etc. I don't want it to be the center of a studio setup - that's not my main usage. Usually if I want to digitally record the Montage I won't use another audio interface or connect it up to the multi-channel one I have. I'll just stab into USB.

Hopefully someone with more direct Mainstage experience will be able to chime in and offer something closer to your particulars.