I am now a little further in solving this problem.
It seems that the config is correct, but that the br-guest bridge is unable to route.
When making the same config with any name but br-guest, it works and I can reach my other devices. However, when naming the interface br-guest again, the dhcp and connectivity stops outside the main beryl ax (wifi still works on this interface and gets served IP addresses).
But I need to call the interface br-guest since this is the only interface next to br-lan which populates the client list in the GUI which I use a lot. Can anyone give me a solution that makes external ethernet vlan connectivity possible AND in which the client list gets populated?