My config was more or less identical to yours, and that removed some doubt and let me focus on the right part: Without a wan0.network, the host isn’t bringing up br0 on boot. I thought it had something to do with the interface having an IP, but turns out the following works as well:

user@edge:/etc/systemd/network$ cat wan0.network
[Match]
Name=br0

[Network]
DHCP=no
LinkLocalAddressing=ipv4

[Link]
RequiredForOnline=no

Thank you once again!