I don't have much experience so I won't throw any rocks, but here's my take on your design.
1 - Gantry uprights are too thin imo.
2 - The gantry bridge might be a bit thicker and if you want less flex, I'd do a torsion box instead of the "webbing"
3 - Your base should also be a torsion box on which you attach your rails. Your whole machine sits on it, it needs to be stiff.
4 - If the stepper specs are right (1.8mH), you "should" be able to spin the stepper to 1000rpm which would give you around 200IPM. I spin mine at that speed with an AM882 and 40v PS. My machine rapids at 400IPM with 10mm pitch ballscrews.
Nice job for a first pitch. Took me 2 month designing my second machine.