This was fun. The sprites were nice and I felt that the music fit. I know you said prior it is royalty free but the choice was still a very good one. My run was done using only the regular shot and never switching gears or gun modes/using abilities and I was able to coast pretty comfortably past wave 50 on Normal mode for a final score of 4,638,325 doing nothing but backing up and eventually doing a slow turn circle to sweep in and pick up all of the drops. I did enjoy the idea that different colors of enemies restored different gauges (I assume this is the case. I only got to eventually see that green orbs filled health when I took my first hit at around stage 30).
To be honest I don't know how you can design the game to encourage movement beyond moving backwards and circling since that is an extremely safe and efficient playstyle but if you can I think you have a really good base game from which to build on top of. Thank you for sharing your project!