You’d need those networks on separate vlans, and a switch with support for vlans. The orbi also needs to support this

Could not bind - socket address is in use indicates another dhcp server on the same host.

No matter what type of service you can only bind one process to one port/ip combo

You can only have one dhcp server - but no matter what one it is - it can point to your pxe boot server

Don’t know how this exact system works.

Right - so Link Aggregation is a thing no matter what the others here say.

To use more than one port per link you need some kind of support for it. LACP is quite common (and easy) form of link aggregation but also there are other implementations.

Speed is one thing I question (or pps as you have 2x amount available) but also failover. If one cable/interface stops working the other(s) keeps things working

It would only be vulnerable from inside threats like some other infected machine.

Merlin has - like stock- an AP mode that disables the wan part. I don’t recall if ac66 has/had wireless-bridge.