How are the Wii U servers holding up? I know there are real folk out there boycotting paid multiplayer.

The official servers for all Wii U and 3DS were shut down a couple years ago. There are unofficial servers through Pretendo, but not all functionality has been implemented yet. I haven't personally tried it.

There are unofficial servers through Pretendo

Thank you, that has been noted.

I'm aware that Pretendo exists, but I don't know if it's active enough to get into a full 8-player match at any time, I'm guessing it probably has to rely on Discord matchmaking.

I also feel like it needs to be said that S1's balance is not good, and I'll bet anyone still playing it today knows how to run circles around a casual beginner. It's not like you'll be getting any kind of skill-based matchmaking here.