Acro/rate mode seems to work very well with PIDs, I'm happy to report, just not the same PIDs as our current angle mode works well with.
The simple answer might be to have separate D/F terms that are used only in angle mode (the current ones), and a separate set of tilt rate P/I/D/F(?) for rate mode. From what I see so far, rate-only mode likes a high P, low or zero I, and a much lower D than angle mode. In angle mode this translates to very sluggish/unusable controls (at least with CTRL_PID_TYPE = 0).
I'm not sure if the P/I/D/O Maximum values would need to change between modes, so far I'm not seeing a need for that.
The good news is that there are only half the PID settings to tune for rate-only mode vs. angle mode.
-Max