Huh - didn't expect to find other people with the X73S. There are few of us in general, let alone running dRonin
Anyway, I couldn't get the thing to stop wobbling - getting the P down would decrease the wobble but also make it harder and harder to control. Eventually I gave up and decided to let it do an autotune anyway, applied those settings, and lo, now it seems to fly fine. I'm actually quite impressed.
I haven't yet had time to fly it FPV so I can't say how it compares to the Betaflight tune it had previously, but the signs are promising. More progress!
One more problem, though - hopefully the last: if left on its own it rolls way right. It's as if the roll stick was constantly pushed right, I have to keep it rolled left by a fair amount just to have it stay still. The trims on the transmitter are the same I used in Betaflight (all zero) and the input tab indeed shows the roll channel to center correctly, so I'm completely at a loss.