https://tools.ietf.org/html/draft-ietf-v6ops-cpe-slaac-renum-01 has been stalled out in the ietf for far too long. In fact, I think its recommendations too conservative, I'd like to see privacy addresses expire
slightly less than daily, in general, in the interest of general security and typical usage patterns. Heck, dhcp is usually 12 hours; that's more than enough.
So that said, I don't know what openwrt actually does.
https://tools.ietf.org/html/draft-gont-6man-slaac-renum-00 was the general problem statement (which also applies to client behaviors). I too use slaac on my routers as the every.router.on::1 thing is scary.
Anyway, there was a patch that went by recently for systemd that got rejected because this draft had not been adopted as a standard yet, and I don't have a full opinion, (as I only re-started working in the openwrt universe a month or so back). As best as I recall from the cerowrt (2014!) project we thought half the offered dhcpv6-pd time for valid was a reasonable compromise, but that was years ago. What does openwrt actually default to for preferred and valid lifetimes nowadays?