I'd say there's a zero percent chance that Ron DeSantis would run on a third party spoiler ticket if he didn't earn the Republican nomination.
What about Donald Trump?
Trump wouldn’t run against DeSantis if DeSantis wanted to run, IMHO.
If Trump wanted too, he might ‘endorse’ DeSantis if DeSantis wants it.
DT might want to team up but I doubt that DeSantis would. He knows he’s better off in Florida right now. so does DT. More face time the better, later on. DeSantis is younger, also. That’s a good thing!
Time will tell.