Jup, the rule as it exists now ("No player can play 2 games in a row on the same slot.") will result in the captain and their first pick just trading air and rock back and forth, while the other two play beach and mid.
So if nerfing captains is the goal, this rule won't do much.
If you wanna nerf the captains specifically, enforcing the opposite, i.e. having them only play their choice of beach or mid, would be a lot more effective.
Alternatively just having everybody play whatever spot they like achieves the same balance wise, while also allowing some of the TMM players to compete much easier
(and probably resulting in more interesting gameplay, as you can go all in on the BO practice for a single slot, instead of getting a "good enough" BO for two)