For a specific wifi connection I know that IPV6 SLAAC won't succeed but
DHCPv6 will. So I've set the "Method" to "Automatic, DHCP only" under
"IPv6 Settings".

The interface also gets the expected IP addresses, but routes are no
more set because net.ipv6.conf.wlp3s0.accept_ra is unexpectedly set to
0.

This doesn't make sense at all, because with DHCPv6 routes are still
announced by the routers. DHCPv6 does not even have support for
announcing routes.

So please stop setting net.ipv6.conf.*.accept_ra on the according
interface if the IPv6 method is set to "Automatic, DHCP only".

In case my expected semantic is not the intended one, please label that
option differently (e.g. "Automatic IP via DHCPv6, no routing") and add
another option labeled "Automatic, use DHCPv6".

It's not that NM doesn't expect to need routes from RAs, but rather
NetworkManager implements IPv6 neighbor discovery in userspace, so it's
expected that the kernel's implementation is disabled when NM is
managing an interface.  RAs are still solicited and processed.  See
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/tree/main/src/core/ndisc

systemd-networkd does the same thing.

I'd argue that it doesn't make any sense to apply IP address
configuration without any corresponding routes, as described in your
proposal, but we can leave that to the maintainers to ponder if you want
to reopen this and pursue it further.

noah