I'm going to push back on you a bit over this, although I think a lot of what you said here is correct. You can't win unless you secure your base first. In this respect Trump is clearly the titular head of the GOP.
@SoCo correctly pointed out in another post the GOP establishment hates his guts, but Trump's appeal to voters (particularly among base GOP voters) is anti-establishment, which includes GOP king makers.
Beyond what you correctly pointed out about the country being primarily in the middle, it still also leans center/right IMO. Meanwhile the Democrat party establishment is under the complete control and is in fact prisoner of far Left radicals! They no where near represent where the majority of the country is...even if they tact Right as most Democrats try to do in order to win in the general election (like you said Clinton and Biden did). Democrats in order to win therefore have to lie about their true agenda, and it will take an anti-establishment type opposition candidate like Trump or even DeSantis to expose the far Left radical agenda.
Trump still lights up a room. No one can attract a crowd and fire them up like he does. Despite how popular DeSantis undoubtedly is, he doesn't have the crowd appeal Trump carries, and his conservative bonafides still are questioned by some in GOP circles (falsely imo btw) particularly among GOP primary base voters. I personally think he'd (DeSantis) be a fabulous candidate, but I also do not think he carries enough weight to beat Trump head-to-head for the GOP nomination. Unless he can win the GOP nomination, he won't get a chance to run against whoever puppet the Democrats settle on. I don't think Biden runs again.
So while I think Trump has some work to do, like disciplining his message and staying away from the personal insults & invectives...I still think he's the candidate to beat both for the GOP nomination and in the general. The more radical a candidate Dems pick and then try to hide, the easier Trump wins those folks in the middle you correctly pointed out any candidate also must win in order to win the Presidency.