Hi! Wierd shapes can be awkward to spray effectively. The rule of thumb is to treat panels like a box if that makes sense, try to maintain straight even overlaps for the most part. Ive painted quite a few cars and sometimes still struggle with vehicles that have a lot of curves, because you tend to want to follow them.
A bumper cover for example, that has several recesses for grille and fog light openings etc, can be tricky as well. I treat them as a box while maintaining even gun distance from the panel at all times. Once ive sprayed it "as a box", I go back in and dust in the recesses or details that were missed.
As far as your tailgate, just maintain your gun distance as you move across the variations of the panel. Also keep the gun perpendicular to the surface at all times. So on a panel with that many variations, you will be constantly adjusting your wrist as you move over the different angles. Hopefully that makes sense to you.
I think another thing that may help with that tailgate would be to hang it up and spray it. I think it would be easier because you would have full range of your arm and wrist motion vs with the gate laying down, youre already putting your arm and wrist at an angle to begin with.
Also, you mentioned your fan pattern. You generally are spraying with a wide fan. Start with it wide open and slowly turn it in until you get where you need to be. Usually doesnt take much. Once you get it dialed its usually only an inch of so smaller of a pattern than wide open. Thats just generally speaking. Theres a lot of factors. Make sure you take your time and do some test sprays on some masking paper and gun your gun dialed in.