There is CAN-FD support coming on H7 with the next NuttX update, check these PRs:
I’m not sure if it’s in NuttX 10.3 or 10.4 but some PRs are coming soon here.
That being said, I tried to get a CAN device (not UAVCAN) working on top of 10.4 but I could not see my device using candump in the console. Maybe I’m missing something, or maybe it’s not that trivial to use a CAN device using CAN-FD although I thought it was backwards compatible.