I don't know, and honestly it's a long ways away still. However I don't think he is that stupid. A lot of people make swipes at him, but people really underestimate him. The dude won the presidency in the first place, it's like people calling George W. Bush a moron because of his verbal mistakes when public speaking. They may be dumb for presidents but they are presidents.
If the Republicans nominate Desantis, Trump could burn that bridge and fuck him over, or get behind him and play the "I helped him win" card and make him look like a "Kingmaker" whether he did anything or not. But I think completely screwing a Republican nominee over will harm his following a lot, as well as his legacy, and I imagine both of those are things he still cares about.
Or maybe he'll go Scorched Earth because he really is that stupid... who knows, but I don't think he's really THAT crazy.