Captured the feel pretty well

Maybe add a couple more "hit" pieces, splosions, ammo, nuclear colors. Capture the busy feel that SHMUPs have (aye, even on easy difficulty.)
If I'd point out the most general thing, biggest difference between your mock and the shot from Monolith, it is that you gotta be more specific with your details.
Monolith tiles you see some nice crispy details, the hinges and the knobs in the FG.
Even in the BG you can see a lot of implied crates, frames, etc., and that's only 2 colors, pushing limits.
Whereas your tiles could be a lot tighter, FG and BG.
I know this is only in the early stages, so you will most likely continue detailing as you polish this.
Something to keep in mind.