Ah I didn't realize that you could dynamically scale glyphs with size. And I would use Shift or whatever to active the time slowdown, but I either only notice it for 1 second, or I don't notice it at all.
If I can be frank, I think there are too many mechanisms at play. "snail based" flying and glyph drawing is a novel idea and it can become something really cool, you have the basics of it down. But IMO you should remove everything else. No dodging of enemies, no time slow downs. Just a plane trying to protect the base, which they do by glyph drawing. I would automatically accept any size glyph without the need to manually downscale (larger glyphs make more missles, smaller make less, etc. But accept any size). Different glyphs can make different attacks.
I would watch someone play your game live, and follow what seems intuitive to them. My game got ripped to shreds by trying to add in too many mechanics instead of focusing on the core mechanic. I think that feedback is right for my game, and right for this game.
I hope I wasn't too harsh, I really did enjoy this game. Best of luck :)