All good, I've fixed up the post to reflect 2009 instead of 2019.
Short of it being a mechanical problem (I don't know enough about the D5 motors to speak to this), everything you've described sounds to me like a power issue, as opposed to an electronic problem.
I would take a close look at the main power to the starter, as well as the main ground lead from the block to the chassis (I'm not sure where this will be on a D5 but it will be a pretty fat cable.
Check any connections, bolts, terminals etc are clean and not loose.
If you have a multimeter, check all your voltages and resistances on both the 12V and ground side, and compare between happy and unhappy starter scenarios