Hardened Edge Router Firewall Lan Difficulty

A while back I hardened a Edge Router firewall following some guides online.

Now I’d like Syncthing to discover the other machines locally on the LAN network without undoing all of the hardening, just adding one or a few specific rules or exceptions to do that but I have no idea where to start?

I read in the documentation I need to allow/forward broad-/multicasts for local discovery to work.

Can this be done with a single exception to the rule for port 22000 on the local lan?

How to keep my LAN secure, avoid hard-coding IP addresses in these PC’s and enable the local discovery feature to work on an Edge Router?

Any help would be so much appreciated.

Everything you need is listed in the docs, not sure we are in a position to give advice how to harden your network.