Accessing sonos devices through wireguard on openwrt

Hi,
i have a router running openwrt and acting as wireguard server. I use my android phone as wireguard client and I am able to connect to the router and see the devices on my LAN. The only issue I have is that my SONOS app doesn't find my sonos speakers.
During debugging I found a few articles, including this one: Sonos fully operational across VLANs (Updated Solution) - #2 by Blowfly
In my particular use case the 'vpn' and 'lan' interface are on the same zone. I am using avahi and mcproxy to reflect messages between the two interfaces. With avahi I can already see chromecast devices with the spotify app. However my SONOS app still doesn't find the speakers
I didn't/couldn't add the firewall rules from #5 in the article linked above, since both interfaces are already in the same zone - I assume this makes it unnecessary to add any additional forwarding rules between them.
According to all the forum threads I found so far my implementation should be working. Does anyone know what else I could be checking? What other information can I provide?

Is forward for lan zone ACCEPT? Then these rules are not necessary.