What is the opinion of Reddit about the
4K HDMI USB KVM Extender TCP/IP Over Single Cat5e/6 up to 120M(390ft), [email protected], Plug & Play, Keyboard & Mouse Over Network, Zero Latency, 3 Ports USB Hub, DIP up to 16 Sets?

There is such a thing as network KVM switches. There are risks in routing the traffic in through a firewall, but I suspect it could be managed.

This is the one I use: 4K HDMI USB KVM Extender TCP/IP Over Single Cat5e/6 up to 120M(390ft), [email protected], Plug & Play,Work at Home, Keyboard & Mouse Over Network, Zero Latency, 3 Ports USB Hub, DIP up to 16 Sets https://www.amazon.com/dp/B07N2M5YYR/ref=cm_sw_r_cp_api_glt_i_3TPFGBJG039SX55BFJS5

I use a usb to stereo dongle in one of the ports, and that gets me keyboard, mouse, hdmi, and sound.

You could use something like this: https://www.amazon.com/Extender-Keyboard-Ethernet-Network-Latency/dp/B07N2M5YYR/

Then you would only need to run ethernet cable between the rooms. I have no experience using or maintaining these things, so I can’t speak to how they work in real life. This particular one is just the first one that came up when I tried to find ethernet KVMs.

This is HDMI+USB but this company also makes HDMI only. This has worked flawlessly for me. https://smile.amazon.com/dp/B07N2M5YYR/ref=cm_sw_r_cp_awdb_imm_EPA8HFDN46TZWRHT9PXP?_encoding=UTF8&psc=1

You want this and to run a Cat 6 or Cat 7 cable from downstairs upstairs.

Why do you need USB3? Lots of USB2.0 options. Unless your wife specifically needs USB3 (VR headset?) I would stick with a HDMI/USB balun like this.