I run 25 meters AOC HDMI, the problem was to pull it through pipe in the floor to the living room - those connectors are quite bulky. Some AOC cables come with mini-hdmi and, an adapter to the full size, but it wasn't available for HDMI 2.1 at the time. Works flawlessly. To switch between display on the desk and living room I run a small DIY java app in windows (shouldn't be a problem to run it on Linux), MQTT + home assistant as a remote app. I didn't play with CEC.

More interesting is a USB setup at this distance. I picked Ethernet - USB 2.0 converter and a simple USB hub with external PSU in the living room, $30. This enables BT, xbox360 dongles, keyboards. I didn't go with USB 3 as its expensive and unnecessary.

EDIT: It's easier to find under 'usb extender over ethernet', and I double check mine ATEN UEC260 costs closer to ~100$ now, way more expensive compared to what it used to be. It requires a dedicated CAT5 cable, it cannot go through any networking devices.

Depends on what you mean by “proper” and the exact layout you are trying to work around.

I can state from experience that drilling a hole through a wall, installing brush plates on both sides to make it look neater, and passing display and input cables through, works pretty well and costs very little. I was using wireless input devices, but still had input cables through the wall with the other end of the wireless link plugged into them, as the range limits of the devices' radios was problematical otherwise.

If you sometimes need to use the machine in its own location as well, then you need a screen there with the pair set to mirror the same output and a local set of input devices, sharing/switching audio output might be a touch more faffy.

Less practical if the device and screen are not near enough to the joining wall, or are in rooms that don't share a wall, of course.

With usbip you can run a USB device remotely as if it is local. You could use this to, for example, access a printer remotely (Wireguard would also allow you this). I went for fiber in my home, through the walls mostly. Also still some legacy twisted pair (esp. PoE).

I think the options are dependent on your setup. For example if you have a smart TV running Android, you could run https://www.virtualhere.com/usb_client_software on it to connect a dongle or controller attached to the TV through to your main device. I do this with my Nvidia Shield and Xbox One Controller.