Most standalone APs can be plugged into the router and immediately start working, they’ll forward along DHCP requests. You can turn off your router’s wifi after they have been configured. For Unifi APs you only need the controller running when you want to manage/update the APs and for stats collection, I only power mine up to check for new firmware updates once a month. Can disable Unifi analytics/telemetry with a config file option too but no way to do it via web UI.

For VLANs you will need to configure the VLANs on Opnsense and the APs. Unifi lets you specify the mgmt VLAN and VLAN per SSID. For my setup I have vlan 5 for work ssid, 10 for mobile devices, 15 for IoT and other things that don’t need internet, and 20 for a couple temporary & guest SSIDs.

The Unifi APs are alright but the controller software itself is fairly limited for stats/data, still better than other standard consumer APs I’ve used though. I’ve been wanting to try out Grandstream Wifi APs for replacement as most models include a built-in controller capable of managing more than enough APs for my home uses and still have option of standalone controller or cloud managed but it’s not priority as my current APs still receive firmware updates,