Oh! Yeah wow I guess this is a Unity limitation... but I have a solution!
Do all your quads use completely different textures? If you make it so the quads get their textures from the same file, that should save on submeshes. (STM will combine these quads to use the same submesh) So... if you were printing emojis, putting multiple on one .png instead of each in their own .png is what I'm getting at. It shouldn't be too hard of a switch to pull off, I think!
Let me know if this works, because otherwise I think this would be a hard Unity limitation...