I’m currently playing it on my Steam deck. Works great.

You do you, of course, but I can’t imagine enjoying playing RDR with a keyboard and mouse. The marionette controls are already bad enough with a gamepad.

Yuzu will let you map your controls this way all the same.

Dolphin is a GameCube/Wii emulator and only runs GameCube and Wii games

For this instance, you need something like Yuzu or Ryujinx that can emulate the original Switch hardware and run the games made for it on PC