It only needs one port, but for most simple networks two ports on the router means less configuration.

The "router on a stick" paradigm using VLANs to a share a single physical port is perfectly valid. You're creating a "now you have two problems" scenario in which you need a VLAN-capable switch and have VLAN configuration to make.

I typically like the ISP router on a dedicated router port to make monitoring the physical link and/or cycling the physical link easier.

Unless your ISP is >1Gbps adding a second port to most devices is as easy as adding a USB NIC.

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.