I've only been doing it for a year and change so we'll see how it goes, but I think it's a good approach.
For example, read this and follow the links: https://calnewport.com/case-study-how-i-plan-to-study-for-my...
Obviously ignore the stuff that's less relevant for an autodidact, though seriously consider the effect any particular thing could have on your learning. For example, perhaps you'd get a high ROI paying a history graduate student to assign and grade a research paper or exam.
Ali Abdaal also has great suggestions that should be useful: https://aliabdaal.com/the-essay-memorisation-framework/
My tentative thought (and I haven't validated this entirely) is to try to structure it. Make a spreadsheet with tables for people, events, etc. Look at Wikipedia infoboxes for inspiration into the types of things that should go as columns in the tables.
You can also try hierarchical periodization. Like if you were making flashcards about the life of Peter the Great you'd divide his life into:
1. Early life
2. Grand Embassy
2.1. Austria
2.2. The Netherlands
2.3. England
3. Great Northern War
3.1 Start
3.2 Founding of St. Petersburg