I come from more of a progressive pop writing perspective, so bear with me a bit; I know your genres are typically less complex or chord-based.
>Coming up with melodiesIn my experience, it's easier to write an interesting melody when you have a starting point — usually a chord progression. Chord progressions that seem tonally ambiguous (as in you can't exactly pinpoint the exact key it's in) or just use modal mixture in general tend to have more ground, so to speak, to let you come up with something more interesting. Once you have that foundation, everything else gets much easier. You can also try experimenting with multiple distinct melodies at once, going in and out of harmony with each other (this is called counterpoint). If keeping the chords in makes the arrangement sound too dense for the type of music you're trying to make, you can always simply the chords into basic 5ths or 3rds — or delete them entirely. Melodies built from arpeggios sometimes work, but it can be formulaic.
>LayeringI generally stay away from direct layering and have each instrument play a different part (layering for sound design though is fair game). Even having chords and then arpeggios built out of those same chords can be too much. I would try to focus on the distinct line each instrument is playing and how these lines interact with each other in the grand scheme of the track. FL Studio makes this easy because you can put the MIDI for every instrument in one pattern and see them all at once.
>StructureI used to have the same problems with arrangements but I got around it using a few tricks.
1. Experiment with sections or loops that aren't just 4 or 8 bars. Try 6 bars, try 10 bars, try 12 bars. Other weird counts that aren't multiples of 2 can also work but sound a little weird (but that can be used purposefully as an effect).
2. DON'T have the song repeat the same general loop over and over again even IF adding and taking away layers. Experiment with having several distinct sections in one song (kind of like a verse-chorus-bridge thing). I try to establish a melody/line in the first section, develop it in the second section, subvert or contrast it in the third section, and then bring it back home by repeating the second section. Although experimenting with other structures also works well — just make sure it doesn't get stale and stays moving or changing alongside its natural development.
>Music TheoryKnowing theory can be useful, but if you have good intuition and instinct, you can brute-force many good songs regardless (ex: The Beatles). I do think it's still useful to know and can help you understand what you need to do to develop a song to its maximum. Here's a good free online resource for that:
https://viva.pressbooks.pub/openmusictheory/
