QT - If I was to give you script notes.

Rating: 5/10 (29 September 2019)

Quentin, I love what you have done here. This script sings a love for Hollywood that leaps off the page. I like that you're changing history again and I think that this mixed genre thing you've got going on could work really well.
I do have some issues however.
One of the things I look forward to in your screenplays are the hidden coincidental connections and clever foreshadowing of events to come. e.g - "Garcon coffee!" or "Pai Mei taught you the five point palm exploding heart technique?"
I know you really wanted to do 'a day in the life' of these characters, but the lack of connections between these stories makes for some uninteresting set-ups that never quite pay off.
1. Steve McQueen sets up the relationship between Jay Sebring and Roman Polanski, but we never see or hear from him again.
2. Bruce Lee and Cliff Booth fight, but we don't get a resolution?
3. If the Manson Family gets killed before they murder anyone, Charles Manson will never be implicated in the murder and he will still be out there.
4. Jay Sebring is clearly trying to get back with Sharon and Roman is cool with him hanging around all the time?
5. The Western TV show with Rick was cool, but he goes and acts in a Sergio Corbucci film we never see?
Now, I'm no Oscar winning writer, so take this with a grain of pepper...
What if you scrap the TV Western scene and convert it to a sequence in Italy showing Rick working with Sergio Corbucci on Nebraska Jim.
Nebraska Jim is a success and when Rick comes back on a high, he finds an invite in the mail to Sharon Tate's party.
Rick and Cliff Booth head to the party, and as they walk up to the gate, Bruce Lee and Steve McQueen pull up in a red Ferrari (I am sure you know this, but they were good friends in real life. Bruce was his trainer and Steve was an accomplished martial artist). Bruce and Cliff have words before they speed off up the driveway and into the party.
At the party Jay Sebring spots Rick and they have that interaction you wrote at the end where they talk about his body of work, showing Jays appreciation for Rick as an actor. Sharing their mutual love of pop culture, Cliff, Rick and Jay become friends and make some drinks together.
Steve McQueen and Bruce Lee chat to Sharon Tate when Roman Polanski comes bursting in pissed at Jay for trying to steal his pregnant wife.
Outside the Manson family members (Tex, Pussy, Sadie, Patricia and Linda Kasabian) pull up with Charlie in the driver's seat. They all smoke acid dipped cigarettes (except Charlie, who pretends to).
Charlie runs them through his plan once again, making sure everyone knows what they have to do. They get out of the car and Linda Kasabian has second thoughts. She makes a run for it, leaving them with a part of the plan missing - Charlie now has to go in with them.
Jay and Roman are about to get physical. Rick and Cliff stand on the side of Jay, and Steve and Bruce side with Roman.
It's about to get nasty when Sharon's stands in the middle trying to break them apart. They start to fight, Sharon's water breaks and she goes into labor.
BANG! A gun fires in the air and five Manson family members (Including Charlie - who is holding the gun) walk into the party.
Charlie gives one of your famous monologues about his distaste for the entertainment industry, and the Mexican stand-off begins.
In an awesome Kill-Bill-esk kung fu fight, Bruce, Steve, Rick and Cliff take down all the Manson family members.
Only Charlie remains. He's had the gun the whole time, but has shown his true colors and been too afraid to use it.
Bruce, Rick, Cliff and Steve all begin to converge on Charlie. He who knows he's done for, so he turns the gun on himself and pulls the trigger - CLICK! he's out of bullets.
In a Death-Proof-esk ending, Cliff, Rick, Bruce and Steve take him to pieces.
Sharon gives birth, and Roman and Jay make up.
In a post credits scene (because they're all the rage these days) we see Rick, Cliff, Steve and Bruce working together on Game of Death - The movie Bruce Lee never finished.
Just a suggestion.
Hope it goes well for you.
Peace.