I used Plex for years, and it is the superior product (if you pay) compared to Open Source alternatives. However, after seeing Plex’s recent incentive pivots and looking for investors I jumped shipped to Jellyfin. The thermometor of enshittification is indicating that Plex is on its way out.
Folks who haven’t looked at alternatives yet, do so now.
No need for masquerading… that seems way too involved. Install Avahi or the UDP broadcast relay plugins then configure those.
As for firewall rules you’ll need to allow mDNS UDP packets (port 5353 and destination address 224.0.0.251/32)