FrogReaver
The most respectful and polite poster ever
The thing about the bonus actions from Shield Master and TWF is that they're just attacks. You can make a shove, or a weapon attack without your stat bonus to damage, as part of the Attack Action. (I think most of us would let a player nerf his own attack if he wanted to.) That means that it doesn't matter, at the time the dice are rolled, whether it is a bonus action or not.
There are so very many ways to get advantage in 5e, having almost permanent advantage from a high athletics is not overpowered. It eats your bonus action for the turn, and your dice can (and will) betray you. It isn't guaranteed or free... you took a feat for it, after all. Anyone can shove, the shield master can just burn a bonus action instead of an attack to do it. No big deal.
This discussion is not one about whether it's overpowered. Why even bring that up?