The problem is I run my own DCHP server (mainly because I have stuff like PXE booting set up).

I can configure the ISC dhcpd for IPv6 but I wouldn’t know what prefix to use in any automated way. So whenever the modem disconnects/reconnects, for whatever reason, I then need to somehow manually update the DHCP server.

Not an issue for ipv4 with NAT. But enough of a problem with IPv6 that I gave up on it. However I do accept that this is a problem of my own making (ie not using ISP provided equipment).