>I've spent many hours learning to play guitar and ukulele but I'm really not very good, and probably never will be - but I can hear the music in my head I want to create. I'm not interested in monetary gain at all, just being able to hear it for real and maybe share it with some people.

Luckily there are hundreds of MIDI editor DAWs available. Open the piano roll, and write down the music note by note. Surely, it's not that hard if you can hear it in your head.

If you are very good at music theory and have good pitch recognition absolutely, but that is a skill that takes quite a bit of effort to train.