Hello. Thanks for playing! I'm using SDL for this game. I actually think the base is kind of wonky but I can actually see all the terrible code and small weird bugs that don't affect the gameplay enough to be noticed normally (thanks for calling it solid anyway). I've been trying improve the art, especially the background art, for a while, but I'm scared I'll just spend all of my time on that instead of working on the game and then have it be bad anyway. But it used to be ten times worse, however impossible that may sound, so I think I'm making progress. I agree with what you said about the characters. They are very WIP, especially the guy, since until the last version he didn't even shoot. I was actually planning on giving them different bombs, but it's not very high priority compared to the other stuff. Ultimately the idea is for them to complement each other, but I've also considered making them completely different shot types.
Making the difficulties actually isn't that hard. Most of it is just changing a variable or two. I didn't plan on having them initially but I added them because people were dying too much and I want people who aren't good at shmups to be able to finish the game too. Anyway, thanks for all the detailed feedback. Thanks for the shmup recommendation, too; I'll check it out and see if I can take anything from it.