I ended back on the vendor messages app, too, when every other FOSS option failed to manage group messages correctly. The worst was one that looked fine on my end, but to everyone else in the group it looked like I was sending out 1:1 messages to each of them. This particular group it a mix of Android and iOS users, and the Android users saw it funky, too, so it wasn't just an Apple thing.

I guess SMS messaging is just a lot harder to get right than most.