Just finished my first credit, and as a Gradius V fan, loved it, it's already so polished and enjoyable. Went to the stage 6 boss, but died stupidly. Will definitely try again and follow your work !
The screen legibility is very good, which is essential for a good schmup, apart from two things that bothered me a bit: The green mechs that arrive in the background, their color is not dark enough to convey that they can't be hit. I got accustomed to it, though.
And in the lava stage, sometimes the foreground and background are too similar, I think.
The idea of displaying the speed level is also such a great one. Maybe a similar system to convey the weapon level for double and laser could be nice. I'm just nitpicking here, hope you don't mind.
Just FYI, I played on Linux via Wine (Nobara and Proton GE latest if some want to know), had zero problems.
You have a very solid game on your hands, keep at it :)