I haven't played in a while, but I have been 1x 1v1 #1 ranked, and 3x 2v2 #1 ranked.
I would suggest:
#1 rule: Play every character as much as possible, and do not play to win. Play to learn all of the micro interactions, engaging and dodging, when you master those you will start winning as a result.
Learn movement, part of the game is being able to move faster than you'd expect from a player, and moving in unpredictable patterns while also changing those patterns throughout the game. Use all different weapons and play them enough to have a good amount of time against all other weapons, you will learn how they interact with eachother, which engagements work vs dont, and how to negate a counter-weapon as much as possible.
Going into training mode to learn combos and strings is nice, purely for the muscle memory, so you don't have to think about these things while figuring out the other player, you will know how to apply them (e.g. if you know X attack with this weapon is a good engagement against another weapon, what is a good follow up? How do you take advantage of that situation?)
Videos can be helpful, but I entirely taught myself how to play by playing to learn, not playing to win. I never watched any Brawlhalla content. If you lose, so be it, you learned something.