When I used to write games in 6502 assembly, I'd use a more native style rather than modeling the code after a high-level language. For example, I'd maintain global settings instead of passing lots of parameters, often having these live across many function calls. I wouldn't pass more parameters than fit in A/X/Y. The main loop was sometimes a big run of inline code instead of having many routines that only get called once. And so on.

Nice project! Looks like it was fun to work on.