[Dagger2]

You don't need prefix translation to use a ULA prefix. You just configure both the ULA and the ISP-delegated GUA prefixes simultaneously.

[rnhmjoj]

Right, the ULA prefix theoretically has lower preference, so it should only be selected to reach hosts in the LAN and the GUA for everything else, but I don't know how well softwares handle this in practice.

[Dagger2]

Source address selection is usually left to the kernel, so that part should be okay. It'll pick a GUA source for a GUA destination unless you've changed the labels with `ip addrlabel`.