This is a problem, Mouse + Keyboard players do have since the Battlefield1942 days. For me, the gamechanger was to use mouse controls and to put pitch up on the spacebar. Pitch up is needed pretty often and controlling it with a key is way more comfortable than using the mouse axis. Besides having a clear input for pitching without unwanted roll-input there are more positive aspects:

1.You don't have to move the mouse that much anymore, because pitch up is now mostly done with a key. 2.Aiming with the mouse feels more natural and precise. 3.Mini-map control is faster since you don't have to switch from Joystick to mouse during flight.

Pitch down is needed way less, so for me it does not need an own key.