Have you tried looking for an official openwrt build for your Airport AP’s? Flashing that would give them full VLAN and multiple SSID capability. Everything a unifi switch can do plus more.
I’m running 1 tplink tl-wr940nv6 as an access point and another tplink tl-wr841nv9 as a repeater for both of my wireless networks. The DMZ vlan starts on an extra gigabit nic coming out of the pfsense box, and going to a second port on the tplink router, that is tagged for that vlan, effectively using the tplink router as an advanced switch and wireless AP.
I paid no more than $40 for both tplink routers. You can repurpose just about anything with the right software. Openwrt IMHO is a more professional piece of software than what the unifi AP’s run.