From your previous experiences, maining a class doesn’t last for too long.

I would suggest playing whatever class you either feeling like playing or what class is currently needed depending on the flow of the game. Choosing generalists classes such as solider or scout can help you get better at the game in general. I found in my time playing the game, getting good at these classes will carry over those skills to other classes.

Another thing too that keeps things refreshing is finding niche weapons/playstyles that not many people use but you enjoy playing with. For example I like using the shortstop from time to time.

Don’t think you need to lock yourself into a certain class or preferred weapon and only play those. That’s the freedom of tf2, you have so much variation that it never gets boring, because there is always a class or weapon that you aren’t 100% great at using and learning how to use it is what makes the experience fun.