If you set up resources correctly on his netbird network it effectively is a split tunnel. You don’t have to route anything through it but his “file shares and stuff”.

I have a rather gnarly nested double nat within my house where the homelab is isolated behind it’s own separate firewall and have some limited resources on both. It’s been perfectly serviceable.

I believe I have a resource of *.whathaveyou.net on the netbird gateway inside the network and no default route. If you do have that default route I believe you can have it as a separate resource and disable it.

Self-hosted netbird is pretty incredible to be honest.