Good, still viable router (providing it’s on the wire). WiFi is pretty well borked.
Dual partitions allow you to keep both OEM and OpenWrt to reside on the same device (change back to either/or).
There is a Community Version Here and OpenWrt forum here
Yes. Use current build.
It would help if you could articulate your use case.
As a first step, I would follow the Quick Start.
By default, OpenWrt will pretty well work OOTB on the wire.
Go Nuts. Nothing to gain other than a secure environment.