Front and flank attack, movement, offensive, surprise.
That’s the way to attack a force in the cavalry, and that’s what Custer did. As US general in chief Nelson Miles and Confederate cavalry general Rosser said, the plan was good.
The front columns didn’t do their job, and the flank column, with Custer, was left alone.
You’re ignoring the obvious point that Custer had plenty of time to form an opinion on the reliability of Benteen and Reno, and then he developed a plan contingent on both of them cooperating effectively with this attack.
A commander’s most important function is to understand and use the men he commands effectively. If Benteen and Reno weren’t up to this plan, Custer should have developed another.
It is pretty well understood that Custer’s only concern in planning the attack was to prevent the Indians from escaping. That part worked. What didn’t work was that he was unable to handle them once he caught them.