Just gave it a few tries and produced a(n unlisted) superplay video:
I was hoping for no miss, but I get tagged trying to find a better shot angle on the final boss like a real dunce. I don't think this reduced my score, though.
From your description it seems like you may be done with Neon Diver, but if you want to return to shmup design one day, here are review-ish notes I've taken from my play.
Gameplay:
Neon Diver is easy, simple, and short. None of these things are bad. Simple and short is kind of a necessity for a hobby project, even.
But it's also fairly passive, which is not ideal - in my opinion, anyway. There are lots of moments in my superplay video where there's nothing for me to do because there's nothing on screen to interact with, or nothing to change about my current strategy because it's going to keep working for the next 20 seconds.
There are ways to solve this without increasing the difficulty. Take the multicolored bullet waves that I back off from in the middle of stage 2, for instance. If the player had increased firepower width and if enemies dropped pickups (even just score pickups) on death - these two things sound like they'd make the game easier, but a superplay would be forced to either swoop back and forth to collect pickups, or feather the shoot button to avoid defeating enemies when they're not directly above. As it stands, I just sit in the bottom-center of the screen with the shoot button held down instead.
You can also force the next enemy formation to come in the very moment that the previous formation is destroyed, which would remove most of the "nothing on-screen" downtime. This has little effect on players that aren't confident enough to hunt enemies down quickly like I am, but gives me more opportunities to score (or die) as I rush through more formations before the boss's scheduled spawn time.
These examples are just things from the top of my head that I've seen in other shmups (Crimzon Clover, mainly), but I'm sure there's plenty of other ways to keep the player busy without making the game too hard.
Oh, and there's no mercy invincibility on respawn, which is a little strange.
Other:
Neon Diver's main omission as far as audiovisuals go is "a sound that plays when you're hitting stuff". The orange explosion sprite stands out nicely, but it's best if I can tell when I'm shooting something without even looking.
Due to how few different bullet/enemy sprites there are, sometimes enemies surprise me by having more HP than I expect, or bullets surprise me by acting very differently from other bullets in the same attack formation (the first boss's first attack is the only example I can think of off the top of my head right now, though). Just a palette-swap would make these things a bit easier to understand.
The look and feel of Stage 4 is my favourite part of the game. The big change in music and background make it clear that it's the final stage from the moment you start, and the bullets coming in from offscreen later on is a simple and effective way to sell the feeling of "approaching an extremely dangerous weapon from afar" and also play on my nostalgia for the older Touhou games that liked to use the "bullets with no apparent enemy shooting them" kind of thing.
Finally, some bugs that I noticed:
The pause menu doesn't seem to actually let me quit, so I've just been using the top-right X to restart my runs.
The bomb button seems a bit finnicky? I've been making sure to let go of shoot and double tap it when I need something behind me destroyed. Might just be user error.
If you're aware of how to do the final phase of the final boss and go for it immediately, you can overlap the cutscenes (as demonstrated in the video)