Wii games are not inherently more intensive that GameCube games. The example I always give is Twilight Princess (GC) vs. New Super Mario Bros. Wii. And there's also (I think) Rogue Squadron 2, a GC game which is currently one of the most intensive games to emulate.
The wiki contains a lot of information on game compatibility and any potential issues. I suggest checking it; it'll likely contain the answer to your question (which games have issues).
"Top quality" is vague and doesn't mean anything. 1080p should be reachable for most (if not all?) games with your GPU and with the DX12 backend, but keep in mind that the DX12 is still experimental and can have bugs. And yes, DX12 is only available on Windows 10.
The wiki contains a lot of information on game compatibility and any potential issues. I suggest checking it; it'll likely contain the answer to your question (which games have issues).
"Top quality" is vague and doesn't mean anything. 1080p should be reachable for most (if not all?) games with your GPU and with the DX12 backend, but keep in mind that the DX12 is still experimental and can have bugs. And yes, DX12 is only available on Windows 10.