In theory odhcpd should handle this. According to @slh it's even a major reason why odhcpd exists: What is the role of odhcpd / dnsmasq ？ - #22 by slh
However, I never got dynamic prefixes working with OpenWrt, sadly.
I had a similar setup you describe and I can remember that I wrote an ugly script that restarted the WAN6 interface when the IPv6 address of the Fritzbox changed (which was polled via TR-064). That was really messy but did the trick. Unfortunately I don't seem to have this script anymore.