I had routing set up on the main router:
config route 'switchWireGuard'
option interface 'lan'
option target '192.168.9.0'
option netmask '255.255.255.0'
option gateway '192.168.15.2'
But it didn't work. Only after adding the nftables on the switch, it did.
I'm not sure what you're doing on the switch... normally this would not be required unless you're specifically filtering things there (which is an edge case).
But that said, it sounds like you have a solution, so that's good!
If your problem is solved, please consider marking this topic as [Solved]. See How to mark a topic as [Solved] for a short how-to.