How to serve DHCPv6 prefixes without making the interface have a GUA?

To simulate a "real" IPv6 service, I'm running OpenWrt on a VPS which tunnels its GUA /56 over WIreguard to a router at home.

This works fine if I manually configure the prefix into the home router. Being a "real" ISP though, it should advertise it on DHCPv6.

Whenever ip6asign is set on an interface, the interface itself takes a prefix out of the pool, reducing the size of the available assignments. This is unnecessary.

I see no reason for an interface that faces downstream (a LAN) to hold a GUA at all (prove me wrong?). The upstream router can be reached as a gateway and DNS server using its ULA. That is how cable modems work (sort of-- there's actually a lot going on inside a cable modem that is supposed to be invisible to the customer).

So I'd like for uchcpd to serve prefixes without the side effect of a GUA being taken away.

The not-working server side configuration that I'm working from is:

config interface 'wg0'
	option proto 'wireguard'
	option private_key 'XXXXX'
	option listen_port '51820'
	list addresses 'fe80::1/64'

config interface 'wg0_s'
	option proto 'static'
	option device '@wg0'
	option ip6assign '60'
	option ip6class 'local'
	option ip6prefix '2600:XXXX:XXXX:XX00::/56'
config wireguard_wg0
	option public_key 'XXXXXX'
#	list allowed_ips 'fe80::4/128'
	list allowed_ips 'fe80::XXXX:XXXX:XXXX:XXXX/128'
	list allowed_ips '2600:XXXX:XXXX:XX00::/56'
	option route_allowed_ips '1'


config dhcp 'wg0'
	option interface 'wg0_s'
	option dhcpv6 'server'
	option dhcpv6_na '0'
	option dhcpv6_pd '1'
	option dns_service '1'
	option ra_flags 'other-config'
	option ra_slaac '0'

Interfaces without addresses, except scope link:
Yes, this is possible, to use only link local addresses on the routers interfaces.
BUT you need to have at least one GUA address on i.e. lo, which can be used as a (valid) source address for everything with destination ::/0, which is not of the scope link.
And you would need to have any Dynamic Routing Protocol, to find other routers and their routes...
This is called IP unnumbered.

People often do not like it, because

then you can not tell from the traceroute via which interfaces the traffic flows...

AFAIK, a DHCPv6 server can use only link-local addresses on the networks it serves directly.
A (central) server, which is also used by dhcp-relays would however need a ULA and/or GUA (to receive and sent), and each switch and router needs to have at least 1 GUA/ULA.

I don't see a reason why you want to allocate/reserver a /60, and not only a single /64 here.

But besides that: How would a wireguard peer be able to use DHCPv6 to configure its interface?