You are right Stephan, automatically add eth1 with DHCP runs into trouble of default route etc but it also depends on how you setup your DHCP for the second network.
In my case I use static assignments and disable default route and DNS settings on my DHCP server, so it works as expected 
On the other hand even if I want to assign a static IP on Incus level you properly hit this issue Troubleshooting Static IP on unmanaged bridge. Back to square one…
I’m not sure how many will run into this or have the requirement it just makes migrating OCI images tricky as most of them have no notion of TCP/IP.
Just thought to bring this to your attention for future enhancement.