[rnhmjoj]

Well, delegated IPv6 prefixes are supposed[1] to be static or somewhat persistent, but some ISPs do this, yes. This is most likely a practice carried over from IPv4 where there is a small pool of addresses. Fortunately in my experience it's not too common: most ISPs that deployed IPv6 did it the right way.

Anyway, to get persistent addresses you can set up a ULA prefix (the equivalent of RFC 1918 addresses) and a simple prefix translation[3]. This is a form of NAT, but unlike the usual IPv4 NAT (actually NAPT) it doesn't deal with ports, so it's slightly less annoying problematic. There also are a few more techniques, like using mDNS and writing firewall rules that match the suffix of the client addresses, but not many CPE allows for this.

[1]: https://www.ripe.net/publications/docs/ripe-690/#53-why-pers...

[2]: https://en.wikipedia.org/wiki/Unique_local_address

[3]: https://openwrt.org/docs/guide-user/network/ipv6/ipv6.nat6

[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`.