I don't think he would stand as a third party - his control over the GOP is based on the threat of that, and if they call him on it (and he loses the nomination) then one wonders whether he'd go to the expense of running an obviously doomed third-party run.
I mean, he could have much the same effect (and generate a lot more money) by not running, bitching from the sidelines / suggesting his voters shouldn't back whoever it is and continuing to fundraise to support "worthy candidates". If they then lose 2024 they will end up coming to him.