So first thing you can disable the camera lag by unchecking these two on your spring arm
You can enable them again later should you want to once we get a good idea of why your movement is spinning out. This is part of the “fun” with physics based movement. It looks like you locked the arms themselves fine.
Yes the tutorial is made so you go forward unaffected by gravity unless you go too slowly.
If you’d prefer I could show you a better way to do this without physics. Are you trying for like plane never stop moving movement or like floating drone full control?