This may not be any consolation to you, but I've understood the movie to be an exercise in Tarantino's personal wish fulfillment. If you aren't on his wavelength, that's always going to read like aimless artistic masturbation. Tarantino is a pop-culture nerd in general, but he has a specific penchant for movies of the 60s and 70s. The Tate-LaBianca murders represented a seismic shift in American pop culture, officially ending the "Summer of Love" and ushering in the paranoid and grimy "New Hollywood" of the 70s. Because Tarantino has such affection for the pictures which largely went out of fashion in the wake of that upheaval, he allows himself (and the audience, of course) to idly day-dream about a scenario where the Manson murders didn't happen, and there were a few more years of this comparatively idyllic period in LA's history. I don't mean for this to be interpreted as a 1:1 equivalency, but, I think the climax of the movie is meant to be the same kind of wish-fulfillment that the audience gets out of seeing Hitler machine gunned into mincemeat in Inglorious Basterds.

I don't remember enough about the craft of the movie to comment upon whether or not Tarantino was completely aping the style of Westerns for the film, but I'm willing to bet there's a large amount of truth in that statement. Tarantino is certainly no stranger to lifting shots and concepts out of his influences whole-cloth. Certainly you could read the title to be a reference to Leone's "Once Upon A Time in the West", but I think more illuminating reading is that "Once Upon A Time...in Hollywood" indicates that Tarantino is telling us a fairy tale. All of the Tate stuff is there to remind people that this was a young woman who was ascendant, all set to become a star, were it not for the actions of Manson and his followers. It's very bittersweet, in that way.

Full disclosure, I listened to the entirety of Helter Skelter, the book the chief prosecutor wrote about his experience with the investigation and trials which followed (despite a general distaste for true crime as a genre), in the weeks before the film's release to prepare. That dramatically recontextualized a lot of the movie for me, because I was keenly aware of how brutal, and how senseless, the real events were. So, I was very willing to join Tarantino in his daydream reverie, and it helped me feel like the frequent diversions to seemingly unrelated plots and characters were all of a piece with one another in a way I don't think I would have without that context.

She was there for one reason and one reason only:

Iirc the real life person she plays was murdered by Manson. So her parts are there to allow her to survive and enjoy life in a way. It's all very meta. I also couldn't get much from the movie. I guess it's more of a vibes movie about a certain time and place that you need to have fond memories of to really enjoy.