5GHz coverage benchmarks? router vs PC?

Hi all!

I'm currently using the modem-router provided by my ISP (CIK, in Québec, for those who wonder), and I'm seriously considering getting a router with OpenWrt with following reasons in mind:
-Better control and monitoring of the settings & usage
-Better 5GHz coverage: I can get it on some devices, but for my cellphone, for example, it's totally unreliable
-Potentially adding Pi-hole and wireguard, though that may end in a separate machine
-No will to spend a fortune
-I'm also considering various IoT small projects, and my own router would make an ISP change less painful...

After going through various threads, it looks like the Xiaomi 4AG would be a good candidate. But I was wondering if there was any benchmark around about the wifi range of different equipments?

I also have a (9 years old) unused desktop computer (that could very well be the future Pi-hole & Wireguard server) that I could recycle for that purpose. Would I be better off with a "real" router or by adding an ethernet port and WLAN addon to a desktop computer?

Thanks very much for your insights!