@spickles Thanks for the explanation. I think I understand now.
Yes, the example I gave is router on a stick.
I didn't dig deeply, but it appears that while pfSense supports VLAN traffic segregation, it does not support tagging inbound traffic onto a VLAN, i.e., no concept of a PVID that can be set per port (excluding the few Netgate devices with built-in switches). Tagging has to be done downstream: a host that tags its own traffic, a switch or an AP.
You might be able to get something similar to what you describe with bridge groups (haven't played with them myself on pfSense) rather than VLANs. IIUC, each bridge group can be configured with its own router config. That way, all ports in the bridge group would share a gateway/routing/firewall configuration. For individual hosts, their port on the pfSense would be assigned to the relevant bridge group. For the multi-VLAN AP, each VLAN that also support hosts directly attached to the pfSense would be assigned to the bridge group containing those hosts/ports. I've never tested whether VLAN subinterfaces can be assigned to bridge groups, but the GUI seems to support it. The downside of bridge groups is that the bridging is done on the CPU; there's a performance hit.