How would you create a brand new exciting sci fi universe?

I'm not sure really. Torn between space opera and hard scifi...

As for your detailed questions, well I can't answer those. It's your universe. Your creation.

An example:  In Orson Scott Card's 'Ender' universe, there is no FTL travel, but there is instant communication. I do it the exact opposite way in my universe. There is FTL space ships, but no instant or FTL radio communication. FTL communication has to be done by "mail" and courier ships.

The rest is just opinion:
No, no flying cars. Overkill. Aircraft though.

Of course there should be gene technology. We have it today. The question is - how successful and popular is it? Maybe it's banned in many factions?

Aliens? Doesn't the story you intend to tell answer that question?

Time travel? No, IMO.  But again, what's the story?

Galactic threat? You know, the galaxy is really big, really really big. 400 billion stars. Don't bite off too much.

How humanity an interstellar power? Do they really remember that? Is humanity ONE interstellar power? How do you think UN is coming along?

Mass Effect, The Culture and MCU are mutually exclusive.

You know, considering what you say about being stumped, I would insist on repeating the advice I've already given. 
Through my own writing process I've looked at this. Different writers have different approaches. Some writers plan out the whole story first. They have their story, or enough idea of what they want to tell. So they figure: How do I break down this into scenes? What do I need to show? They figure out how to tell the story in a set of relevant scenes. They make lots of notes about the scenes, what they must contain, what ideas and impressions they need to transfer to the reader. In the end, they got everything ready, and then they just type out the scenes.

That's all fabulous and people who are good at it, produce impressively well constructed and smartly told stories.

Here's the secret: A lot of very famous and highly appreciated writers do not do it that way! That even includes crime novel authors, and even from the old days of the typewriter. Raymond Chandler, for instance, didn't do it that way. Neither do Rachel Kadish, who wrote the last novel I got really impressed by, 'The Weight of Ink'.

Instead, they have a loose idea. Then they start writing. And as they do so, they tell themselves the story. The characters' personalities evolve during the writing, as you imagine them in your head, and as they interact with other characters. The story may also go off in directions not predicted. Just follow them.

This is how my own writing got going. I write because it's fun, As fun as reading a novel, and having the power of shaping it.
So I urge you to try to do the same. You're writing on a computer, which is perfect for this approach. You will need to go back again and again, and work over the text again and again, all through the entire project. And you must keep a parallel document listing all the characters and details as you go along, for your own reference. And you will change things. Because as you write, things will take better and more detailed form in you head, and you may wish to fresh up the earlier parts to keep them consistent with how the story develops.

But start writing. Don't fret so much about it. Just put one foot ahead at a time and repeat. As you go, ideas will start to come to you. They may not be good, but just keep going. You can change it later, when you get a better grasp of your story.

It's fun to write on the virgin ground, continuing the story. It's not so fun to go back and rewrite. That's a bit of a pain and also slowing you down. But it's easy. As you go back and read, you see what you must do with your earlier text. You don't get "stumped" there. It's just work.

In the beginning I tried to get things as "good" as I could from the start. I still try, but I don't fret so much about it. Because there were two things that happened during the process: 1- I become a better writer. 2- I come to understand the story better. Both are reasons to go back and fix things, even sometimes deleting entire chapters, or inserting new ones. In general, the text tend to swell.
Keeping this experience in mind, I tend to slow down to plodding in details, when I feel stumped, and rush forward when the story comes to me, just sketching, because I can go back later and render it better.

Hope this helps. In short, just sit down and write. Don't obsess with having the story ready. It's not really possible. A novel is such a big and complex thing that it can't be completely constructed before you start to write. It will come to you as you work with it. I have deleted the beginning of all three novels I'm currently working on. Together well over 100 pages. But those beginnings got me started.