I have this chess learning theory
After viewing many tactical position problems, I noticed that many of them were similar. So I started to select only those tactical positions that show one or three postitions for the same tactical theme (fork, X-ray, pin, decoy, etc.).
But, later I found that some tactical problems were counter-intuitive, meaning simple problems that visually are difficult to detect, in effect, not a matter of calculation, but trouble seeing the combination. I would define as counter-intuitive solution, as any tactical problem that YOU have difficulty solving that is less than 3 moves deep.
What I am getting at is this, if one continually selects and re-selects tactical problems that are from different themes and few repetitions and combine the selection with counter-intuitive problems, then one might Not have to study thousands tactical problems, but reduce it to less than 300 tactical problems! Which would really help to just study only 300 tactical problems.
Because the purpose of tactical study, is not so much to memorize the specific position, but to recognize and apply the tactical theme in other positions that almost allow for tactical advantage. In this way, the player is searching for incomplete tactical patterns, and tries to complete and bring about the pattern to his advantage. In effect, his/her chess planning is just that, the search for incomplete tactical patterns.
An example of incomplete tactical pattern can be any tactical problem (with a solution in less than three moves) where one has subtracted one move before arriving at tactical advantage. (I put my tactical problems on index cards and frequently re-select them downward, so that I have a full example of tactical themes and counter-intuitive problems with few repetitions. And I have added cards where there is no immediate tactical advantage because I have stepped back one move.)
One little drawback, this might work well for players with OTB rating of less than 1500. To go beyond, then deeper planning is needed.