Lots of great ideas and the truth of it all is apart from any physical aspect of it, it is making you think about what you are doing.
That's why the fretless took you out of your comfort zone, because the technique needed to be more precise and consistantly precise.
Now the danger is you do not expand on this, you still play the same sort of music and exercises on a fretless rather than get out of your comfort zone and learn again.
Tombone exercise books are a great source of information, as is a bugle book, a bugle only has chord tones so of anyone thinks that is limiting listen to what bugles can do.
Also try working on basic drum rudiments, learn to play different meters and metered patterns and then add melody or harmony to them.
Classic example of this is, Lessons in love by Level 42. Simple use of triads, but getting the rhythmic meter of it is as much the song...not the just the notes.
Working on different meters will bring new skills and understandings but I will take you right out of your comfort zone for a long time so do not feel overwhelmed by it, just a meter at a time. You already know how meters in four work, learn meters in three and you have most of the picture in construction.
Most meters will be combinations of threes and fours, sub-divided down, so if you cannot combine threes and fours to make and play meters you will certainly not be able to sub-divide them down to make other more complicated ones.
Drum rudiments are a great source of this, as are drum books.