I found the information below in the Chess Wisdom Session of the American Chess Magazine [#47, 2025 page 82]. Answers from questions to IM John Donaldson [BTW, I highly recommend American Chess Magazine]:
“Considering the three main phases of chess — the opening, middlegame, and endgame — how should players effectively allocate their study time among them? Which phase do you believe is most critical for a player’s development and why?“
“Ideally one should divide their study time equally between all phases of the game. This is the approach used in the 10th volume improvement series by Artur Yussupov. Published by Quality Chess, it is aimed at players from 1400-2200 and I highly recommend it.”
“All players need to work on their tactical ability and calculating skills. Solving puzzles and endgames studies should be part of everyone’s training program.”