But that's something that they can continue doing. There are many countries where they have a second round between the two candidates with the most votes, and in those countries, parties generally send only one candidate. And that's what they'll keep doing under this system, because they want their candidate to end up in the top-2.

So unless you actively ban any kind of candidate selection by the parties, that is what would also happen in the US.

I believe the particular design of the top-two primary allows anyone to enter the primary and list any party as their "preferred party." So the party might endorse a particular candidate, they can't prevent other candidate from getting on the ballot under their party name.