Oof.

Yeah, because you're gonna have to have a DHCPv6 client running on your router (and because your ISP is almost certainly using DHCPv6-PD the router is where you're pretty much going to have to first learn about your LAN-side DHCPv6 prefixes), it's probably going to be a bit tricky (but probably not impossible) to do what you want.

Best of luck. If you figure out how to do it within the HN comment freeze period (I think it's 14 days?), please do leave a follow-up comment. I'd be very interested in hearing what you come up with.