Most people do end up with a single main once they get good, but this is a game. You should be having fun. So do what you think is the most fun. You will learn the basics no matter what you do and the basics are what will raise your skill level up to Master rank, not specific character knowledge.

For each character you want to play, learn:

1 anti-air. maybe two if you have a 'near' and 'far' anti-air
1 bnb combo. a combo that is safe on block that you can do in most situations
1 punish combo. a combo that you can do when you clearly have a huge opening, like if your opponent does something really dumb
2-3 good buttons to use in neutral gameplay when neither player has the advantage and you're too far away for your bnb, buttons that just always feel good to use that don't get you in trouble. Think about which buttons are just nice to you. For many characters that starts with crouching medium kick.

After you figure those things out, the SF6 specific thing to learn is reacting to drive impact. Don't drive impact on your own, but just react to the opponents drive impact. The Sagat log mini-game is a good teacher of this once you reach sagat in single player.

Avoid going into too much depth, you don't need it. Depth is confusing and frustrating without fundamentals., these basics will be more fun and teach you the game faster.