The room target isn’t really meant to represent just AC. In fact, the room target works best when you contextualize it as something mostly immutable, like the environment, or as a function of the narrative. It’s not meant to simulate real-life, it’s more a way to easily control tension and catharsis in your game. A target of 12 is fairly simple, and allows players to breath a sigh of relief, while a target of 16 might be too high to handle, and signals that maybe that a change in plans should be considered.
Making monsters distinct comes down to 2 things: their abilities, and their decisions. How hard they are to hit isn’t as important as if they can cast a big AoE spell, or heal themselves. If you have a boss NPC with a handful of henchmen, then you make them distinct through other, more nuanced mechanics. Maybe the boss reduces all damage taken by 3, making them much tougher. Maybe the henchmen only have 1 hit point, so they can be taken out en masse. If you’re a patron, I might recommend the podcast on Monster Sets, where Hanks talks about making enemies distinct through their mechanics, and how they can inter-relate to each other.