Not a Unifi guy (mostly transitioned to pfSense these days) but, a fail-over needs a fail-back. If your primary WAN has gone down and you're failed-away, there needs to be a way to periodically probe the primary WAN, revert back to it once it's working again. Also, load balancing? And there are sites you might want to preferentially route via your secondary WAN? Depends on whether you're paying per-byte or per-connection (potentially with a speed limit). I have no idea if Unifi can do this, but probably?

I don't think I'm your target audience anyway - sorry to interrupt. :-)

Edit: it also seems to me that you might need VLAN-aware switches(?) Those are unusual for residential gear here in the US.