By some miracle I built a legacy kernel 5.10.160 with RBD support. I can't say exactly what exactly helped, but I did the following:
1. I literally added output/config/linux-* and config/kernel/linux-* to every file:
CONFIG_BLK_DEV_RBD=m
2. Added to ./config/boards/orangepi5-plus.conf
MODULES="rbd"
3. Repeated previous build instructions.
And on this kernel the OPie5+ boots without problems every time.
