Laszlo argued that calculating 10 moves deep is useless if you fail to see a simple knight fork in 1 second. His middlegame PGN groups positions by pattern. For example, a single file might contain 200 "Knight Forks on c2/f2" problems. After solving 50 of them, you will never miss that fork again.
Buy the ebook Chess: 5334 Problems, Combinations, and Games (Laszlo Polgar) on Forward Chess. This app allows you to export positions to PGN from your "Training" tab. While tedious, you can curate specifically the middlegame chapter (Chapters 3–5). Laszlo Polgar Chess Middlegames Pgn