I'm going to disagree with all the sentiments in this thread and recommend something different.
Go rent an Alexa.
For practical purposes, maybe an Alexa Mini. Talk to your local rental houses and see if there's a timeframe you can rent one and get a big discount, often rental houses are happy to give you a discount if you're renting it when the camera wouldn't be rented by anyone else so have a chat with them.
Shoot with it a lot. Shoot as much as you can and in as many situations as you can. Just get one lens with it then take it out and shoot. Shoot in the various modes it has, shoot into the sun and away from it. Shoot indoors. Shoot high-key and shoot low key.
Then take the camera back and grade the footage.
I suspect you won't do this. It's expensive and a cinema camera like an Alexa is a PITA unless you have used one before. So I'll skip to the end with what I think you'll find. The footage won't look great. The footage will remind you of footage from lesser cameras. You will wonder what happened and if you're processing the footage correctly.
I have never shot with an Alexa, but I am told by many pros that if you don't know what you're doing, Alexa footage will look just as much like a home video as from almost any other camera.
Cinematic is a word that doesn't even really have any meaning in this context. It really just means 'of the cinema' and there's probably been enough films shot and shown in cinemas on iPhones that now an iPhone technically qualifies as being 'cinematic'. Yes, i'm being slightly tongue-in-cheek here, but the point remains that the word doesn't have any useful meaning here. Yes, images that are shown in the cinema typically look spectacular. Most of this is location choice, set design, hair, costume, makeup, lighting, haze, blocking, and the many other things that go into creating the light that goes through the lens and into the camera.
That doesn't mean that the camera doesn't matter. We all have tastes, looks we like and looks we don't, it's just that the word 'cinematic' is about as useful as the word 'lovely' - we all know it when we see it but we don't all agree on when that is.
By far the more useful is to work out what aspects of image quality you are looking for:
Do you like the look of film? If so, which film stocks?
What resolution? Some people suggest that 1080p is the most cinematic, whereas some argue that film was much higher resolution than 4K or even 8K.
What about colour? The Alexa has spectacular colour, so does RED. But neither one will give you good colour easily, and neither will give you great colour - great colour requires great production design, great lighting, great camera colour science, and great colour grading. By the way - Canon also has great colour, so does Nikon, and other brands too. You don't hear photographers wishing their 5D or D800 had colour science like in the movies.
What lenses do you like? Sharp? Softer? High-contrast? Low contrast? What about chromatic aberation? and what about the corners - do you like a bit of vignetting or softness or field curvature? Bokeh shape? dare I mention anamorphics?
But there is an alternative - it doesn't require learning what you like and how to get it, it doesn't require the careful weighting of priorities, and it's a safer option. Buy an ARRI Alexa LF and full set of Zeiss Master Primes. That way you will know that you have the most cinematic camera money can buy, and no-one would argue based on their preferences.
You still wouldn't get the images you're after because the cinematic look requires an enormous team and hundreds of thousands of dollars (think about it - why would people pay for these things if they could get those images without all these people?) but there will be no doubt that you have the most cinematic camera that money can buy.
I'd suggest Panavision, but they're the best cameras that money can't buy.