I think you should complete the campaign, you most likely still have two weeks before the season even starts. I am not sure about this, but i believe to unlock freeplay for your seasonal cahracters you have to finnish the campaign

Seasons are a way to let all players start at the same point at the same point. They only thing carrying over from your non seasonal characters are the shrines of lillith that you found as well as your capaing finnish.

You don’t need to play seasons, you don’t even need to play seasons in day 1. You can join whenever you feel like you want to start a new character.

Seasons aren’t a big deal, they’re usually just to offer people an excuse to keep making new characters, to give them a small reason to (like a new mechanic or new build options, items, etc)

If you’re not done with your original character, just keep playing it. You’re not missing out on anything

The fact that season are starting so early into the lifespan of the game is a little odd. Most players aren’t finished with their first character yet

Seasons are a regular reset with everyone starting mostly from scratch. For hardcore competitive players it’s a way to compete on leaderboards and such. For more casual players, most seasonal games incorporate some new hook or mechanic into the game, sometimes only for that season, other times retaining them in the core game.

If you’re playing through the storyline, I’d say stick with that and don’t worry about the season.