It really depends on your objectives. If you want to compose pop music (including rock and electronic music) the risk is to lose yourself in complicated harmony

It really depends on your objectives. If you want to compose pop music (including rock and electronic music) the risk is to lose yourself in complicated harmony "rules" that are mostly irrelevant to the style you want to play (on top of my mind parallel fifths prohibited in classical theory but they are commonplace in rock/metal) and only useful to analyze classical pieces. Personally as a guitarist I kind of hacked my way into music theory to being able to improvise over and compose pop/rock/electronic by only using intervals and scale degrees. I can hear chords progressions, I know why a V I cadence works, can identify and play in modes (dorian, lydian...), but I can't name the notes so transcribing into a DAW is cumbersome but at least I can compose freely. Not ideal but considering the minimal time I invested it's a very good return for me. Concretely the idea is to focus only on patterns and formulas. The steps would be to learn to play and hear intervals, then construct chords, e.g. Major chord is 4+3 semi-tones, minor is 3+4, another +3 is minor 7 or +4 for major 7. Then by using a scale (e.g white notes on the piano for key of C major/A minor) it's easy to find the chord on each degree. After that learn a few common chords progressions and being able to identify them in everyday songs should come naturally. Finally practice composing melodies by targeting chord tones and you are ready to start composing. I found this video does a very good job at providing this overview: https://www.youtube.com/watch?v=rgaTLrZGlk0 https://www.youtube.com/watch?v=rgaTLrZGlk0 (edit: the second half of the video).