A little too dry.
The game was just fun enough to stick with until I beat it, but there's just nothing about this game that really sets it apart from other shooters I've seen. Everything in the aesthetics department is just... very bland, dull, and unmemorable. It does fit with the whole "early shooter" vibe, but far too much so. I note that you do make some concessions to modern shooting gamers to try to set this game apart from the pack, but... Well, let's run down the list.
Being able to buy, pick and choose which weapons you want to bring into battle is a great way to allow players to adjust how they play, but the way you implement the shops, it doesn't really work out. There is no reason to ever, at any point, buy anything but the highest-powered engine and armor available to you, and you present them as single incremental upgrades after every boss. There is also no need to wrack your brain deciding which mod to pick, especially once you reach World 4, where the omni-mods fall to the same tactic of "just get the latest one after every boss". That leaves you with your weapons...
It just feels like there's only a couple of weapons in the game that are really going to do you any good. Early weapons are made obsolete very quickly, and the gimmicky nature of later weapons, coupled with the fact that they're just so much more powerful than earlier ones and the fact that many of the gimmicks just aren't very useful (Here I refer to the pulse cannon that doesn't even shoot more than 1/3 of the screen's height) while others are far more useful (I'm looking right at the gun that pierces through any enemy), means that you'll probably pick a few "best" weapons when you hit the mid-to-late-game and never look back.
I'm also not sure that the meshing of old-school hitboxes with new-school bullet hell, life bar or no, was a particularly wise move, because with many types of bullet spreads, you're simply too large a target to squeeze between bullets, most of which are pretty damn fast to boot- and don't get me started on the dick moves you pull with enemies spawning either on top of you, or coming in from BEHIND with absolutely no warning other than the "where the hell did this guy come from" collision damage, far more than once in the game.
Overall... this game needs a greater sense of identity, or a serious tightening of the game mechanics and shop system, or better yet, both. I know that naming it SHMUP was meant to tie in to the acronym you gave the game, but right now, the game's name feels sadly much more like an indicator of its plain, generic nature than anything else.