HEX as a "tunnel" to connect to IPv6 PPPoE? (EDIT for more details)

Hi everyone,

silly question:
Got a situation where I have to work with UniFi hardware, and the ISP is providing DS-lite via a PPPoE IPv6 connection.
UniFi only supports DS-lite via DHCPv6, SLAAC or staticIP, but I need to stick to their gateway (user wish).

Idea now would be to implement a HEX as a connector to the ISP and the UniFi Gateway as the router/firewall.
The HEX would connect to the WAN via PPPoE/IPv6/DS-lite and receive the related IPv4. For the LAN side, the HEX would be assigned a fixed IP (e.g. 172.21.0.1) on one of it’s ETH interfaces, the Gateway another fixed IP (e.g. 172.21.0.2) on it’s WAN interface, and a static route would be configured.
So far so good.
But what happens with the IPv6 section? I assume (but don’t have a confirmation from my ISP yet) that after a successful PPPoE authentication, the HEX WAN side will receive an IPv6 segment much like DHCP with prefix delegation. How can I forward this to my actual DHCP running on the UniFi appliance?

And are there more things I might have missed?

Thx in advance!

EDIT: more info on IPv4 / IPv6