Beginner ~Intermediate player trying to improve

Hello, I started playing chess when I was 12 in Cuba. After learning the rules and reading some kids books, I joined a chess club two blocks from my house, which I went to every day. A year later, I had an OTB rating of 1400. After achieving this, we moved to the US, and chess stopped being a part of my everyday life. Now, I'm 16 and am picking it up again. I have a Fide Master rated 2378, who coaches me every day over the phone for 1-2 hours and who is helping me as much as he can. I currently have a lichess rating of 1600 in less than 25 games, which is about 1000 in chess.com. I'm starting to take the game really seriously again because I have the time and I enjoy it. In addition to my daily sessions with my mentor and doing some daily puzzles from the book "5334 Problems, Combinations, and Games" by Polgar, I want to train like a high-level chess player would, but of course at my level and as a hobby. I'm very patient and willing to put in the work. My mentor wants me to focus on the middle game now, since he says I have a solid understanding of endgames and openings are not important right now. What would you guys recommend? I want to do more!

TLDR : I started playing chess when I was 12 in Cuba and quickly achieved a rating of 1200. I stopped playing for a while but have recently picked it up again and am now being mentored by a Fide Master. I have a lichess rating of 1600 and my mentor has advised me to focus on my middle game. I'm very dedicated to improving and am looking for ways to train like a high-level player. What suggestions do you have for me?

PD: this is a hobby, I’m not trying to become a GM but have the skills to win local tournaments and enjoy the game at a high level!!!

Play slow time-control games for improving, and fast time-control games for fun and practice.

Play over your losses and be sure you understand why you lost each game. Get help on the forum if you don't understand how or why you ended up in a bad position.

Forget about studying opening lines. Below Expert level, the most important reason for choosing a particular opening is that you enjoy the types of positions arising from it.

Study the games of the old masters from a hundred years ago, like Morphy, Steinitz, Tarrasch, Mason, Reti, Nimzovich, Breyer... stopping perhaps at Alekhine or Botvinnik. Modern play (by players such as Tal, Fischer, Kasparov, Carlsen, Nakamura, etc) does not stand alone. It is BUILT on the play of the old masters, so learn THAT first.

Keep training with you friend that will be best

No need for a tldr for such a short text

@1

"who coaches me every day over the phone" ++ Strange over the phone.

"5334 Problems, Combinations, and Games" by Polgar ++ Good.

"focus on the middle game"
++ Yes, but you cannot really study that. You can play and analyse your lost games.
You can study grandmaster games.

"I have a solid understanding of endgames" ++ I doubt that

"openings are not important" ++ Correct

"I started playing chess when I was 12 in Cuba"
++ Study Chess Fundamentals by the great Cuban Capablanca