Thinking of getting some new carbon wheels for my gravel bike (Sram Rival 1x hydro), which I’d also use on a disk road bike (with Shimano ultegra hydro brakes). Does anyone else do this, and do one set of rotors play nicely with both systems?

The rotors don’t care.

You might find you need to readjust the brakes as the rotors may not be exactly in the same place.

May require minor Caliper adjustment and also indexing.

Shimano & Sram have compatible cassettes and can be interchanged, disc rotor’s of the same size will also be compatible, Shimano or Sram