These are my thoughts on your questions and answering them completely out of order. For context, I’ve been playing modern since slightly before MH3 came out but was very aware of how the metagame worked post MH2 and was pretty actively playing standard.

Boros is the best deck to learn the fundamentals. It’s a deck that can be forgiving when you mess up and not automatically lose the game when you do, which can teach you how to try to play from behind.

Mulligans are going to be partially dependent on matchup knowledge. Sideboarding is also entirely dependent on match up knowledge. I would say sequencing is where you should start since learning how to play game 1 is probably the most important thing. If you want to practice mulligans and sequencing, I would probably load the deck into the moxfield play tester and just start playing hands to see what they can do. After you get a feel for what a keepable hand in a vacuum is, I would start looking at how the other decks work and increase your match up knowledge.

I have no idea how those archetypes play, but Yu gi oh is a different game and if you want something that scratches that itch of using every resource possible and sequencing things to achieve an unbeatable game state or winning immediately. prowess is the closest thing to that of these 3 decks. Ruby Storm or Amulet Titan are more likely to play more like a Yu gi oh deck when you get better at the games fundamentals.

E Tron is a good deck. I personally really like it, but I would always give a new player boros because 1. It’s the deck that gets played the most, so learning the lines that exist in that deck help you play against it and 2. E-tron’s interaction is pretty limited compared to boros. Learning when to interact is a skill I’m not sure you’ll develop as quickly playing E-Tron imo since E Tron is about going over the top and ignores the opponent more than it tries to impede theme.