do you have any advice about dealing with the feeling that your group might be plateauing? we've had creator on farm for several weeks, and we've moved on to speedkills, and i have concerns that some (not all) members aren't interested in improving further than where they are right now

My advice? Don't force players. Not everyone enjoys speedkilling (especially in AST meta) - don't let it destroy a group. In my experiences, interest is not something that's going to spawn out of practice. It either exists or it doesn't.
Side note: there are other ways to further group synergy and unity... Sometimes it's even accomplished by playing other games together. In our (now-deceased) team Updog, we played more D3/BnS together than we did XIV during farm. It's important that people enjoy what they're doing or they're most likely not going to perform well.