From my limited knowledge of other Republicans, they probably fear his influence and reach. The thing about Trump, though, is that every time I read a recollection of some high-level conversation between him and another qualified official, he always seems clueless. For example, in a 2021 interview, Fauci said that Trump would believe personal anecdotes from "friends" despite Fauci explaining multiple times that anecdotes are less trustworthy than scientific studies:

It was always, “A guy called me up, a friend of mine from blah, blah, blah.” That’s when my anxiety started to escalate.

https://www.nytimes.com/2021/01/24/health/fauci-trump-covid.html

He's a puppet because he lacks media literacy and lacks an understanding of logical reasoning, and as a result, he's easily manipulated by other people with stuff like conspiracy theories. Republicans might fear him because he will win. I fear him because he will make bad decisions not purely out of malice but out of a lack of due diligence and understanding of the effects of his actions.