Silman · Dvoretsky · Nimzowitsch · GM Smirnov
2500 is grandmaster. No app alone gets anyone there, and I won't pretend otherwise. Pair this path with game analysis and a coach, though, and the ceiling is high.
Stop donating pieces. Build board awareness.
Tactical pattern recognition. The first taste of pawn structure.
Strategic thinking. Imbalances. Positional play.
Prophylaxis. Two weaknesses. Conversion technique.
Dynamics, deep preparation, and classical study.