yeah we have multiple networks into our house since wife is WFH and they pay for it, we have 1 gig fiber and backup 1 gig cable ($75/mo each) plus a backup Wifi router on Verizon mobile, so I can split people across them, the QNAP has two network interfaces which are hooked to different ethernet switches and you can direct Plex users to different port servers to spread load.
sadly upload speed is the key stat for sending videos over the net, transcoding 4k down to 1080p is probably your best bet but newer synology servers are supposed to be really good transcoding machines. You could do something whacky like mirror Plex servers remotely using rSync but I think thats getting a bit much.
The other option would be to download both 4k and 720p rips of everything, I do that for a few things that I absolutely love. Like I have a massive 585 gigs of Lord of the Rings + Hobbit 4K Dolby Vision extended edition rip, the 720p rips are ridiculously tiny (under 20 gigs) and Plex automatically will serve the right version - 4k locally, 720p remotely or mobile. Same with Godfather, Goodfellas, Star Wars, Rambo, etc. Fortunately most older TV shows are all limited to 480p anyways, nobody needs to see Hogan's Heroes or Looney Tunes in 4K.