I do most of my gaming on a Retroid, which can run a lot of PS2 games well, as well as obviously earlier games. I really liked the GBA Macross game - you got to level up ship stats after each stage to make it more durable, or your weapons stronger, and shit like that.
What other shmups have similar mechanics? Dont care if theyre horizontal or vertical. I already have Radiant Silvergun's Saturn port loaded on my device, so inb4 that one.
Depending on how far back you want to go: Solar Winds 2: Galaxy was a favorite of mine as a kid. It's as open world as you could get in 1993, but I remember loving it even though I didn't understand everything (I was 6 years old). Ship leveling, unique NPCs, and shooter elements as well.