The problem with RPG discussion isn't RPG theory at all. It's about general devolution through social media into a positions over interest culture. Folks want to win, they want to be right, and they want these things above a good discussion. It is also accentuated by gamers propensity for being anal retentive and overly pedantic.
One problem is that gamers, in general, want their games to be perceived as "art," especially when they are perceived as not being "art," but then they get toxically hostile when people criticize said games as they would "art." There is a pervasive sense, IMHO, that gamers want to remove the games, media, and entertainment they consume from any and all criticism. People like what they like. But if they like it, it's good and it
can't and/or
shouldn't be criticized. That attitude may be the greatest hurdle, IMHO, when it comes to developing TTRPG theory:
"Don't criticize games that I like!"
Video games are a slightly older art form than RPGs and have a fairly well-established set of terms and theory. We could do worse than adapting those to suit our purposes. But most RPG people seem eager to reject those terms and theory.
As you say, RPGs are not a monolith, more like 2d20 things stacked on top of each other in a trench coat pretending to be an adult. Likewise, gamers are not a monolith and want different things from the same games. But to make things even worse, the same game can be multiple things, pull from multiple art forms, and need multiple sets of (often contradictory) terms at the same time…to describe one thing.
RPGs are not a monolith. Neither are video games, which I suspect have a greater breadth of play than TTRPGs.* Theory is nevertheless being applied to video games. It's a common error in academia to treat one's own field as being "exceptionally unique" when compared to other fields.
* I mean no offense here to TTRPGs. There is simply far greater number and variety of video games out there than TTRPGs. Moreover, TTRPGs may have more shared commonalities with each other than video games do. TTRPGs are simply a niche hobby.
To circle back, we could do a lot worse than borrowing from video games. They have spent a lot of time and money developing their terms and theory. There’s a lot of overlap and several obvious places where there are pot holes. So it should be a fairly painless lift and shift.
It's a starting point. Often academic fields borrow terms from other fields that have already developed them: e.g., literature -> film - > television, etc. Those terms are then tweaked or discarded when their applicability to the new field or medium is "tested" through criticism, discussion, argumentation, and theory.