Here are some changes I would try in Cura.
Reduce inner wall speed from 80mm/s to 60.
Reduce wall acceleration from 2500mm/s^2 to 1000.
Turn off outer walls before inner walls. (This should put the under run after a travel move inside so it's not visible on the outside.)
Turn retract before outer wall on.
Increase retraction minimum travel from 0.8mm to 2mm.
Reduce Retraction Prime speed from 30 to 25
Find the hidden setting for compensate wall overlaps and enabled it
Find the hidden settings for skin overlap percentage, infill overlap percentage, and skin expand distance and increase them a bit.
Here are some changes I would try in config.g
These may not be feasible with the flex3drive, since I'm not sure what their recommendations and limitations are.
Increase E jerk from 12 (that's quite low)
Increase E accel from 150