4 Uarts and 2 Ethernet possiblities

[gops]

2. 2 ethernet(s) [100 Mbps is fine], one natively available, what is the best option to add second one?

Concerning a second NIC, I use a USB 3 Gigabit Ethernet adapter, it works quite well. To configure it, one needs to add a yaml file in /etc/netplan (in ubuntu)
LMM.

Thanks for great suggestion martinayotte and LMM. 

Any recommendation as module for USB to Ethernet, instead of we connect USB to Ethernet from outside connector, I mean, we would like make a daughter board with a small PCB with host 4 RS 485 (with SC16IS752 x 2), 1 Ethernet USB port. Any concerns like how  RockPro64 help with power supply all ports.