Eww, good luck.
I played Smash 4 on the Pro controller, and I'll play Smash Ultimate on the Pro controller. Over the course of Smash 4's lifetime I've seen many of my friends move from Pro to GameCube controller, but despite the trend growing increasingly prominent I never quite understood it, I find the Pro controller to be quite objectively superior, its only downside being potential lag at bigger events. Of course, with the Switch Pro controller sending data through USB, that's no longer an issue, so there are no downsides anymore - and that's not even getting into how good the controller is as a whole, I absolutely love it. I can see it being a little weird to get used to playing Smash on it at first, because of the right analog position and the size of the face buttons (a notable thing for me, because I use everything on face buttons: attack, special, shield and grab), but I'm sure it'll come good in the end. If anything it can't possibly be worse than the overrated GameCube controller.
Yea itll be a challenge to play with a single joycon...maybe relive the days when I played Street Fighter on SNES controller (but tinier lol)
I agree that the gamecube controller is a bit overated as the better controller for Smash but I guess some people love using it, possibly due to the nostalgia and that those people played Smash heavily on the Gamecube. Personally I think Nintendo's Wii U pro and Switch pro controllers are far more comfortable for me to use to play Smash.