The late middlegame/early endgame is by far my weakest point.
I often get some kind of advantage in the opening, but then struggle to convert. I do study endgames, for example I am currently reading Silmans Endgame Course. But the point is that these kind of endgames that you can learn by rote memorization/technique (e.g. say Lucena Position, or K+P vs K etc.) is not what I struggle with. It's rather to find the right plan in positions, where there are many more pieces on the board.
I try to give an example by some positions that I encountered in tournament games, e.g. longer time controls. I hope this explains better in what kind of positions I struggle.
r1r3k1/ppp2pp1/2q2n1p/4B3/8/2Q4P/PPP2PP1/3RR1K1 w - - 0 19 1. Qxc6 bxc6 Bxf6 gxf6
I noticed by exchanging pieces, that black will end up with several pawn islands and double pawns. But I failed to convert. Later
8/p1p2p2/1rp2k1p/4Rp2/1PP2P2/P6P/6P1/6K1 w - - 0 29 1. Kf2
instead of Ra5, forcing on exchanging rooks, or attacking both the a and c pawns. I simply didn't have a proper plan here and what to do - so even though I can calculate, I did not know what to calculate. In this game black managed to ourmaneuvre me and I could barely hold a draw.
8/1p1k1p1p/2n1p1p1/p1P5/1pK1PP2/1P6/1B4PP/8 w - - 0 36 1. g4?
here I played g4? instead of seeing that I can invade with Kb5, and then Black is stuck with defending his a-pawn so that his Knight is almost not moveable. After Na7 I lost instead of securing a draw.
3r4/8/2Rnk1p1/7p/1P1p1B1P/2p2P2/5P2/6K1 w - - 0 40 1. b5??
which Black can answer with Kd5 and wins. I played b5?? instead of getting my King in the game with Kf1, which would have rescued the game.
In these positions it seems crucial to find the right plan before calculating anything. Typical endgame books kind of don't bring me forward I feel.
What kind of learning ressources and or methods can you suggest to improve in such kind of positions?