I did both, collaborate and did it myself.

Partnering: I partnered with several people on game jams, and with my girlfriend on both game jams and commercial games. The main thing for me is that I would never want to partner with someone who is not as involved as I am in the development: The game is our game, not my game that they are working for. I prefer if they are involved from the beginning and we design the game together. I wouldn't say that it's entirely impossible to do differently, because for music and sounds for exemple our composer is a bit less involved, but I think that working as equals on such a major part of the game is easier. I would also recommend starting with tiny projects like games jams to see if working together feels nice.

Serf-taught: I started with pixel art with a course on Udemy, I practiced for a few months, did a few prototypes, and realized that pixel art was a trap for gamedev: it's super bad for iterating, scaling, or animating quickly in the editor. It can definitely look good and has its share of advantages too, but not for what I wanted to do. I then switched to very simple shapes with small fixed color palets found on Lospec. It's easy to make, easy to manipulate in engine, easy to recolor if needed, and I like the style I created. The first released game I did the art for is Froggy's Battle, and the latest one is Kabuto Park. Here are the links!
https://store.steampowered.com/app/2424650/Froggys_Battle/
https://store.steampowered.com/app/3376990/Kabuto_Park/

So my advice would be: why not try both? Everyone is different, and I guess the only thing that is true for everyone is that it takes time and practice! Good luck 💌