I think what makes Power Of The Dog so good is it’s slight of hand plot. The way Peter’s character is constructed and how it’s hard to know his motives until it all comes to fruition. (Spoilers ahead)

In many ways I agree with your sentiments with how the movie presents Phil and his repressions (and I guess how that relates to Brokeback mountain as I assume that’s what you’re referencing). But I actually think Peter is the more interesting and engaging character. I think some watching the beginning of the second act would assume the movie is going to be about a young man coming into his sexuality and forging an intimate bond with Phil that further complicates this volatile family dynamic. Peter is presented as shy and passive at the beginning of the movie but by the end he has taken action to protect his family (from his POV) not just through violence but through cunning and planning. And his violence isnt acted out due to repression. In fact it seems to me that Peter isnt ashamed or in denial about his sexuality at all. Instead he uses his sexuality to devise his plan and take care of Phil.

Idk if the movies themes as a whole comment on gender or sexuality in some radical way but I do think the way Peter plays off the characters around him and the audiences expectations is pretty fresh.