I would indeed give the router and FMC some distance from the other FMC and DAC… out of the music room would be awesome if practical. I would also leave a little distance between the final FMC and the DAC itself (i.e. not set it on top) to minimize the chance of interference.
The fiber optic cable can be something crazy, like a mile long! As always, your ears are the final arbiter!
My first TPLink FMC is next to the modem in my office, the other end 25m away by the stereo. The latter is battery powered. Ethernet into the server and ethernet from the server to streamer. So no switch or router in the signal path from the modem, and server and streamer are ultra low noise. Works a treat.
Just keep as simple as possible and as low noise as possible.