Well a raspberry Pi 4B with a decent USB3-Gigabit-ethernet dongle (and the proper configuration) should allow for that.
I believe this post might help? That whole thread seems relevant....
No idea, but the R4S package seems nice, price-wise it is close to a RPi4B (with case and USB dongle)
pros: second ethernet via PCIe (possible with the CM4, but )
cons: only 2 instead of 4 A72 cores (I am not sure the 4 A53 cores can make up for that, at least SQM is more latency bound then throughput bound)
What ever you end up using, would be nice to post your final decision and how well it works... Thanks in advance.