Honestly, I would probably just use the Wii for Wii and GC, and then find a different solution for the other stuff. Retroarch setup does have a somewhat steep learning curve, but it's extremely easy once you get past figuring out where the settings are and how to save things per core and per game. You can also save the setup files and reuse them if you have to make any changes
If you don't want to use your PC, I would still recommend a Raspberry Pi solution over the Wii for other stuff because a 4 works great and you can get various 24 bit interfaces out there; the RT Ultimate was also open-sourced by Mike, so you can easily have one built by PCBWay or JLC.
You can compile Retroarch inside of Raspberry Pi OS and run it from the command line
At the end of the day, you're going to have to either setup Swiss or setup Retroarch on something, and IMHO, RA is the more powerful solution both in terms of device agnosticism and video output options