The donors are just going to be the figureheads of these companies, right? That's already how it tends to work...

Tesla/SpaceX didn't donate to Trump's campaign, Musk did. It wasn't Palantir, it was Peter Thiel. (to my knowledge but I honestly didn't check the dono rolls, just going off remembering headlines here)

Either way, the outcome is basically the same. If they ban companies donating, CEOs will donate with a wink wink, as the cost of the donation is peanuts to the profit they'll make. These aren't your standard donations for tax-writeoffs (though I'm sure it helps, too), these are purchases of influence

My pops used to work for Lockheed, and every couple of years he would get a big bonus, then tapped on the shoulder that it was 'his year to donate' to PACs. They'd let him keep enough to cover taxes plus a little extra, but it was understood why he suddenly got a large bonus. This was back in the 80s, so maybe things have changed since, but I'm sure whatever regulations have been put in place are easily avoidable. The people who wrote the laws are the same ones taking the bribes.

If you're suggesting "The good guys just need to out-donate the bad guys", the unfortunate reality is the bad guys are donating because it makes them money, so they can afford to. Nobody bankrolls good deeds that lose money.