how much agency is beneficial is how much do the players care about their decisions mattering. I played in one game where I didn't have any agency. It was one of those stupid dark gritty the world is ending and the railroad did not bend. So me and the rest of the players decided we were hero's and we rufused to give up. We died, we rerolled, we died we rerolled till the DM gave up. We never accepted his view of how we were supposed to act. We refused to give up the agency he was attacking. Never forget no matter how much the DM tries, they can only take your agency if you are willing to let them. Just walking away from the game is agency. Agency can be surrendered it cannot be taken. many a game has blown up over the idea that agency was something the DM could just seize.Another question is how much agency do people want, and how much agency is beneficial? You could maximize agency by just having every player write a story about what their PC does. Wouldn't really be a game, but with no constraints whatsoever it seems to maximize agency.
In other words, even if people agree on how to calculate amounts of agency then there also has to be a discussion of whether more agency, however measured, is always a good thing.