The candidates were decided long ago by a few big donors and the media. At least we have a chance to vote for one of their choices, but even that election cannot be trusted.Two things to note. First, you're right that big donors heavily influence the Democratic nominee. They influence who gets good media attention, and they have their eyes on electability first. If they weren't big players, the Democrats would probably have chosen Bernie in '20.
I don't think they greatly influence the GOP. In fact, I think most primary voters are suspicious of big donors. The donora are represented by the Bush wing. They want tax cuts, deregulation, and defense spending. They prefer open immigration (because it's anti-union) and want to avoid social issues. Nikki Haley is their candidate. Trump is not. He is the product of the voters.
Second, I understand the frustration that Desantis bailed after only one state, but the timing of the primaries isn't the big factor. Had he lost IA but looked strong in other states, he likely would have stayed in. The reason IA, NH, and SC get so much attention is that they are often bellwether states for predicting what other primary voters will do. The problem for Desantis is that looking at the rest of the country, IA looked more like a ceiling than a floor. It sucked, but it was true. Again, this is what the voters wanted.