Some time ago there was a similar thread, only you are in a better position because LAN and not via the Internet.
Another option is to directly connect the PC to the TV with a long cable, probably rather some transmitter(hdmi/displayport over ethernet) that will amplify the signal and the USB (keyboard, mouse, pad).
Of course you will need a separate cable…
Depending on your needs, you need to select the appropriate transmitter. Optionally, an additional DP/HDMI converter on one side.
A USB HUB is best one that also has its own power supply just in case. 
(if the number of ports in the transmitter is not enough)
Or if you want a different approach to long cables then…
Long DP cable as you mentioned and converter to HDMI. However, for USB devices, buy a very long cable / active,
I see such 20 meters at a price of about 30-40,
the only question is whether 20m is enough for you. And then some hub at the end of such a cable and you connect the usb peripherals.
As you can see, there are several options. From software to hardware solutions… Choose what suits you logistically / technically / financially. 