I finally got comfortable tapping out a few advanced polyrhythms: 5 hits over 3 beats, 5 over 4, 7 over 3. My previous method for learning a polyrhythm like 7 hits over 3 beats was to subdivide each beat into 7 parts and hit on every 3rd of those. It only took me a few years to realize that mentally subdividing beats by 7 is impractical, no matter how tempting.
My new strategy is to maintain 2 steady tempos and ensure the beats coincide where they should. I put both on autopilot, and my only conscious attention is on whether I need to adjust the tempo of the hits relative to the beats.
Consequently, I'm getting good at holding a tempo with my foot while tapping strange rhythms with my hands (or playing on guitar). So I can keep my sense of time with the song in my head while totally losing it in my playing. I can't claim that it sounds any good yet, but it has potential.
The same concept applies to outside harmonic playing. If I can improve at keeping track of the chords as they pass, I can improvise in a completely different direction and then return to more conventional harmony.