Yes, exactly. And if you have two upstreams, there's no single GUA prefix that makes sense to use in all situations. You make your routing decision, then you NAT (er, sorry, NPTv6, which is Totally Not The Same Thing As NAT) to the GUA prefix corresponding with the network that you're egressing from.
If you don't need Internet connectivity, yes, NAT-free ULAs work fine.
That wasn't my intent, but I see how it reads that way now. The parenthetical was me griping about naming, not meant to update the meaning of the sentence. In the dual-upstream scenario, I'd use stateless NAT with a single on-link prefix (GUA or ULA).
Collisions between two private networks is very low was my primary point, and thus NAT is not a thing that needs to exist.