How to cut multiple home movie reels into clips, then reassemble them by theme?

I have several dozen 4-minute home movies that I've digitized. The films were created over a number of years. Each reel includes multiple segments, of different events, in no particular order. I'd like to cut each reel into clips (segments) by theme, then reassemble them into reels based on theme

Themes might be something like: (A) hometown parades, (B) trips to Grandpa's farm, (C) visits with cousins. So reel 7 might include scenes of Themes A, C, F; reel 12 might include scenes of Themes B, C, J, etc.

I'd reassemble them into a reel with all the clips of Theme A, and another of all the clips of Theme B, etc.

I've got a basic familiarity with iMovie, and I've done something similar on a very small project. But I don't know how to organize this much larger project, with many different reels and clips and themes and resulting reels. (Roughly 30 reels, to be cut into roughly short 150 clips, resulting in roughly 12 theme-based videos.)

How can I organize all this? I don't want to just stuff all 200 clips all in one collection (window). There has to be a better way to organize it into projects, media, clips, themes, etc. Maybe part of the solution is to tag each clip to identify its theme and the number of the reel it's from, if iMovie has any way to do this.

I'll be grateful for any advice.