If Donald Trump is impeached, he will be the 13th president subjected to US House impeachment probes and/or the introduction of articles of impeachment in the House, and the third to actually be impeached. If he resigns before the House votes on impeachment, he'll be the second president to have done that.
My impression is that Trump doesn't like being second, third, or 13th at anything. He likes to be -- or at least appear to be -- first at everything.
In this situation, the only thing he could possibly place first in is "convicted by the US Senate after impeachment by the US House."
What if that's what he's angling for?