Technically you can route without isolation, but VLANs are definitely a good idea if you’re using a single port.

There are 2.5 Gbps, 5, and even 10 Gbps USB NICs these days, although 10 Gbps ones are pretty expensive and require really recent USB ports.

I agree I want my local network and my WAN port separate, if for no other reasons than so I can use ssh to get into the router from my LAN with the WAN port disabled.