UPS load - 50W average. 13¢/KW. About $4.8/month

Honestly you’re overthinking this. If I were you, I would Passthrough both the LAN and WAN to the opnsense VM, create a dedicated Linux bridge on one port and assign it to the LXC, connected to your 2.5G switch. Even when you’re saturating your 40M upload, it won’t be anywhere close to saturate the 2.5G link between other devices and the switch.

To answer your original question, the fastest way to link your Plex LXC to your LAN would be to create a Linux bridge and instead of passing through the port for LAN, assign the LAN to the dedicated Linux bridge. Have the Plex LXC use the same Linux bridge.