Because of our garbage voting system, any vote not going to the top two candidates is the same as scribbling over that section of the ballot. If your goal is to not have Donald win, you want to add as many votes as you can for the other of the top two candidates.
The only way a vote for RFK would help prevent Donald is if it were coming from someone who otherwise certainly would have voted for Donald. Even then the ideal is that they vote for an opponent who has a chance of winning.
That's not true. Voting third party helps get them funding and ballot access. If your vote for president won't change anything, then vote for the most utility.
The funding gives them visibility though. Without third parties that people know and might vote for, there would be no additional challengers to point to when arguing for ranked choice voting or anything else. If there are no Green, Libertarian, Constitution voters, then FPTP loses a good portion of its luster.
Additionally, if enough people vote 3rd party, the big two may shift to win those voters back. We saw an interesting situation with the Libertarians and the Republicans this time around where the Libertarians weren't going to primary a candidate against the Republican if the Republican met certain qualifications. If the Democrats lose even a percentage point to the greens in a tight race, they can possibly get that voter back by representing their interests to show that they are also green.
Having said all that, I agree that FPTP is a big problem and is strongly contributing to the toilet bowl death spiral American politics is experiencing.