After a few hours of testing, these are the controls I found most useful for flying. The plane can be controlled by just your left hand, precise movements are for the mouse.
GTA 5 - Next Gen How to Change Handbrake & Controls - YouTube
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Is there a mod to set gamepad controls keys independently for walking/driving/flying? | GTA5-Mods.com Forums
I finally found the perfect custom keybinds for flight controls on PC. What do you think? : r/gtaonline
GTA 5 Keyboard And Mouse Controls - A Complete Walkthrough - MobyGeek.com
Controls for GTA San Andreas | GTA Wiki | Fandom
PC Controls - PlayerUnknown's Battlegrounds Wiki Guide - IGN
Download Switching between characters as in GTA 5 v.2.1 for GTA San Andreas
Steam Community :: Guide :: Grand Theft Auto V - Keyboard & Mouse Controls
gta V(5): how to change controls (WASD-arrow keys) - YouTube