This, though, would be the single biggest QOL change to flight, and implementing it would probably just require changing a single constant. At most, changing that single constant and then changing one mathematical calculation so that there isn't a divide by zero error. Currently the options are ascend slowly (hold jump button) or corkscrew. Just let us fly at all angles, realism be damned.
As far as visual things go, banked turns are about as far as I feel we would need to go with dev time spent on mount movement. Just set a max tilt for each mount, and then while in motion have the mount tilt more toward that angle the longer the turn button is held.