Thanks for the guidance, reading through Extra-ordinary Networking for hints and tips. In my case, IP on an Arm Cortex MCU is in itself a bit of a challenge, so I wanted to shy away from IPv6 as my first run. But yes, I will see about the feasibility of link local IPv6 instead of IPv4 plus DHCP. Thanks @Quinn.

I've read through the various links. In particular, the article "Network Interface Concepts" comes close in section "Interfaces and the Default Route" but it doesn't go into much detail on how the Default Route is chosen. In my case, I'm creating a USB CDC NCM device (e.g. a "USB to Ethernet Adapter"-ish) and this is competing with the Wifi w/ NAT adapter on my Macbook for 'who gets to be the default route today'.