My approach:
Make a project, and do some sound design for some bread and butter sounds on around half of the tracks you have available.
Save the project.
Make beats using those core sounds, and allow yourself one or two additional tracks that you can design sounds on the fly.
Duplicate your sounds across all the patterns in a bank.
Make as many beats as you can using those sounds, and either save them if you are happy with them, but more often just dump them and reload the project at the end of your session.
Most importantly, record the result. Listen back to it, especially after leaving a gap for a few days.
Then when you find your hands are moving automatically without having to pause and think too much, start working over multiple patterns, and figure out, either with or without song mode, how to move between one thing and the next.
You ask:
“What, in your opinion, are the 20% of features/tricks I should focus on which would take me to the 80% of making a decent song?”
My answer:
The basics of subtractive synthesis. Learn to take a waveform, use a filter and filter envelope to shape the frequencies, and an amp envelope to shape the amplitude. That’s it. If you understand how these things interact, you can create a great variety of sounds. Then start thinking about modulation with LFOs.