Admissions is moving in the right direction imo, but while it sort of dodges inflation and ERs it’s still very auspectible to market growth. Thus, as you indicate, maybe potential admissions as % of pop, or as % of pop with reasonable access to cinemas. But I think you’d find the latter two trend downward pretty steadily from the “can only see a movie in theaters” ERs to the “home video era” to the “streaming and peak TV” era.
The beat ways to simultaneously try to account for all of those sorts of factors is to compare to surrounding movies, e.g. “admissions as a % of overall movie admissions for a 5-year window centered on the movie in question.” Although a hard N-year window is choppy, so maybe you instead want to smooth things out by taking an average of surrounding years with weight decreasing with distance.
I don’t personally think you’re ever going to get a more meaningful result than “Endgame was by far the biggest movie of the 2010s era” “Avatar was by far the biggest movie of the 2000s era” Titanic was by far the biggest movie of the 1990s era” etc.