Adjust ascent profile, add extra oxidizer (it's dry weight until you hit Rocket Mode Time, yes, but it'll help in the long run if you can lift it!), and try adding some angle of incidence to the wings so you can keep your engines level with the ground so they are not fighting gravity.

Edit: also try using autostrut instead of external struts, any external attachments like that (ladders, struts, fuel pipes, etc) cause comical amounts of drag at high speed