I understand the desire for a single board solution, but if you end up playing with the current RPI4, this thread might be helpful, it includes the discovery of a "good" usb3 ether interface (TP-Link UE300, realtek chip) like Moeller0 mentions, that uses far less resources than some other types that will bottleneck you. There's a few other good threads are out there on Rpi4 use, as well.
[[RPi4 routing performance numbers](https://]