Add flaperons
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Make flap control surfaces with the input -pitch + roll. Just figure out which wing should have a negative roll value. This input can be used on rotators for custom control surfaces too
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You can do this using Overload.

1. increase control surface area
2. if that misbehaves put the center of mass closer to the center of lift (but not
   exactly on it, rather a little in front of it)
3. if that doesn't work, increase the "liftScale" on your wings with overload
4. if all else fails, you can always use gyroscopes, but they're really fiddly

It's probably a bit too heavy on the nose. Try moving the COM back by adding or removing weight.

Changing wing airfoil can change stall characteristics. IIRC it's not super accurate in SP, but a good example would be the New Wasp, which has entirely/almost entirely flat bottom (despite being a high-speed jet fighter).

You can add more lift by setting "liftScale" on the "wing" section in Overload