To be clear , when I say “To be useful” i really mean “To be viable as a professional”. Theres absolutely nothing stopping a beginner musician from making their own music of a professional quality. Rather what I’m getting at is the skillset of Professional composers is that they often have barely weeks to score hours of full orchestral music, with extensive ‘craft’ experience, and the ability to guarantee to clients the job will get done , on time, and of appropriate quality.
Thats fantastic that your making progress on it. I once had a guitar student who at 16 could play like a dead ringer for Jimmi Hendrix, despite the fact she didnt even know the names of her strings. Self belief is hugely important for creative development, and sometimes people just ‘get it’.
Regarding chords, there really isn’t any chords that ‘dont exist’, but there are more complicated and more dissonant chords and knowing when to use them is a combination of black magic and music theory. The core of it is that chords are largely derived from the harmonic sequence of a note, somewhat helpfully on odd numbered degrees of the scale (1,3,5,7,9,11,13,etc). As you go around that sequence, you’ll eventually encompass the entire cycle (Ie 9 is a 2, 11 is a 4, 13 is a 6, and so on, and sometimes these can just be refered to as a 2 4 or whatever, but I prefer to stick to the odd number series when trying to conceptualize whats happening), however the further you get from the 1 in the odd number series, the more dissonant it gets. The central idea is that these bits of added color are there to reinforce components of scale important to the melodic pallete. This might be important also if your working outside the standard major or minor modes. So take for instance a Lydian mode which is often used as an epic sounding version of the major scale in film scores, You’ve got a sharpened 4th in there. You could ignore it, or you could use a Cmaj7(#11) chord, or possibly a Cmaj7No5#11. Weird chord, but used smartly it (For instance in a film score getting some horns to fill in a slightly muted sharpened 11th alongside the brighter major components) and you’ve got something huge. Theres also a few of the oddball scale chords as well, like the Tonal scale chords, and so on. Again its just about picking the theoretically appropriate colors to pain your picture.
If you can play guitar , I strongly recomend getting a good book on Jazz chord and scale theory. The jazz guys are absolute pros with this stuff. The Jazz guitar training I had taught me far more than I ever got out of my classical piano training in terms of bridging theory to creativity. Oh and fun fact: In jazz you get to improvize in every damn song. Even the bassist.