I think you also need to enable full forwarding to and from wan on Router B. I forgot it defaults to not doing that. Set input, output and forward to ACCEPT on Router B on the wan zone, and make sure you also allow forwarding to and from the lan zone. Router A should be fine, I assume A’s WiFi and LAN is the same?
Basically now, Router A sends the traffic to B but B doesn’t forward it to its LAN. But since we don’t have NAT, A’s devices addresses B’s devices directly, not B itself, and there isn’t any connection tracking happening, so it doesn’t “remember” to allow the ping response back in. If you WireShark this, I bet B is successfully sending packets to A and A’s devices, and A’s packets make it all the way to B but B doesn’t forward it to its own LAN, and it stops there.
Can you post the output of ip ro and ip a on both routers? (Feel free to redact your public IP/ISP stuff if it shows up)