@manawyrm You could look into old Devolo hardware off ebay... They seem to have built somethink like what you need some time ago:
Hotel Schlafgut uses them to connect the WLAN APs via their tv coax cabling. 😉
@manawyrm How about using directional / hybrid couplers to split, attenuate and rejoin TX, but keep RX (mostly) unattenuated? Also directional couplers can be designed to act at impedance matching network.
Also consider the fact, that this could be turned into a multidrop topology, akin to old-school coax thinwire Ethernet.
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