I like the overall vibe of what you're trying to set up, however i do feel that challenging yourself to a single synth severely limited the sonic landscape of your tracks making them all sound similar to each other, and while it can be a good thing in certain scenarios, i think with the story you've provided, it doesn't bode well. Another thing I would like to mention is to not be afraid to experiment with more chords! Chords give the melody a home to settle into, so you could make it sound more complex just by adding chords. Not only the melody, you'll also be able to see what countermelodies can be used, which notes should fill the lower end as a bass, and create harmony and fullness to the composition! It does require music theory, but I assure you, just learning major and minor chords and using them accordingly will already broaden your space by a lot!