There are many different aspects to strategy that's what makes it so mysterious. `My System ` by Aaron Nimzowitch will give you a good framework of the basic elements of strategy, but there are other factors. Various pawn structures and set ups resulting from opening play. Maneuvering of different combinations if minor pieces, etc. Then there is the subject of making plans. Usually, this is based on pawn breaks and improving your piece positions. But looking for weaknesses and learning how to create them for your opponent is also a deep subject. I'm sure other posters will recommend other good books or courses.
Strategy
I struggle in the middle game as well. I've been going over GM level games featuring the openings I play and have gotten better at developing a plan. I'm also studying pawn structures. Look for backwards pawns that would be hard for your opponent to defend or look for weak squares that can no longer be defended by pawns and then work to plant a piece there. The Sicilian Najdorf is a good example of this. After black plays e5 kicking the d4 knight, there is a huge hole on d5. A lot of games revolve around white trying to plant a knight on d5 while black works to prevent that while also looking to play d6-d5 himself. Whichever side can accomplish their respective goal is usually better positioned.
So I've found that learning and improving opening knowledge and tactical skills is pretty straight forward. There are hundreds of videos explaining openings and there are thousands of tactics puzzles out there. However, when it comes to strategy it doesn't seem as simple. So what do you guys do to improve your strategy in chess?