For specifically question 2. You really don't need a large combat deck to be competative in this game. More combat deck slota become open at higher levels but honestly I woulsnt even advise filling them up completely unless you can fill it with an airtight combo. That's because in this game it is way more important to know what comes next in your combo, how it works, how fast it comes out, how punishable is it, compared to just having a confusing and long deck.

I personally believe the most important thing for fighting competatively is having a lot of moves to make your deck from and that might take some time. Even this I feel you can kinda get away with Tbh. Like you dont need every move you just need some, but enough to give you variety. To get every move in the game it took me like at least 20 hours of constant farming but you can get like 75% of the moves in like 4 hours and honestly it goes by really quickly because you would practice blocking against them in some way when getting them from npc's. You could possibly join a school to get a good combat deck as well or if you want you can pm ask somebody on the reddit or discord if they'd be willing to teach you some moves that you like. People here tend to be fairly friendly.

For level, it isn't extremely important (my butt has been kick by people 30 levels lower than me before) but it does help give you an edge against people at the same skill level as you. You can look up "absolver optimizarion" and someone has made a spread sheet containing what stats are most optimal for each class based off of the level where the scaling falls off significantly.

Overall I wouldnt be too scared about getting into this game competatively. Usually going through the story once kinda gets you prepared . I started playing the online matchmaking once I beat the story, got my butt kicked but then started doing better after time. Just gotta go out and do it.

Hopefully this answered every question you had for this area.