Because it was too late

Just "stealing" the Stones involved incorporating them into his armor, and recall that just wearing the Gauntlet is incredibly damaging.

So using his armor to steal the Stones meant that, effectively, he was doomed the moment his did so. Using what was left of his strength to dust Thanos and his army was all that was available to Tony... indeed, it was the ONE path to victory as forseen by Doctor Strange.

There was only one way to win out of 14 million and that was it.

There was no point of doing that.

What Tony Stark did, have a deep bond with his own life. To understand why he did, what he did, we should look at who Tony Stark as Iron Man is.

I'm not sure that stealing stones is the right term, but Stark's ultimate plan was protect what he loved most, he is ready to sacrifice his own life to do that.

Since Loki, Tony Stark was living with the unease of that what will happen next, How to protect the world and all the burdens he carry. He even went on creating Ultron to do just that.

Since he learned about Thanos and what his plan is, Stark focuses his all towards stopping Thanos. you can understand how Thanos lived rent-free in Stark's head by just looking at the conversation he had with Doc Strange.

Strange: Under no circumstance can we bring the Time Stone to Thanos. I don’t think you quite understand what’s at stake here.

Tony: No. It’s you who doesn’t understand. Thanos has been inside my head for six years… Since he sent an army to New York and now he’s back! And I don’t know what to do. So I’m not so sure if it’s a better plan to fight him on out turf or his but you saw what they did, what he can do. At least on his turf, he’s not expecting it. So I say we take the fight to him. Doctor. Do you concur?

So, everything he did then , he had a plan. The gauntlet Thanos grabbed at the end of the Endgame, has similar tech like Stark's armor. nano tech and controlled by Friday/Stark. Thats's how he was able to grab the stones just by touching the gauntlet. His ultimate plan was to do whatever necessary, whenever possible and use however tiny chance he gets. Just before that scene, we can see Doc. Strange reminds Stark that they have only one chance to win. and Stark knew when Strange said,

There was no other way

that he has to do the sacrifice. He accepted it then and made his mind around it.

So, to answer your question, why didn't he fly away,

He didn't have much time to do whatever necessary or to bring the gauntlet to someone else. If you watch the battle they had to do just to bring the stones/gauntlet to Scott's van, almost every member of earth's defense, had to get involved and was barely succeeding, and then failed ultimately. So Stark didn't had much of a probable chance to bring it somewhere else, doesn't matter how fast he can fly. Thanos is a veteran in battle. there was no escape. Stark knew that.

He literally wore the infinity gauntlet, which is near impossible for mere mortal. So in the moment he wore the gauntlet, there was no going back. So Stark made the choice.

Tony Stark is not a kind of man, who lets other people to do what he wanted to do or what he thinks he's better at doing. with his ego, he tries to do whenever he wanted to do something that way.

While hulk has the ability to survive with relatively low harm, he already done the snap and was not in the condition to repeat. Hulk was proven ineffective with one on one battle with Thanos in the beginning if Infinity War. considering both these, either Thor or Captain Marvel was the best choices Stark have to choose. but as I said before, trying to move gauntlet could result in failure.

Tony Stark is not a man who accepts failure. so he chose to solve the problem by himself.

To answer your second question, slow down Thanos.

There was nothing to gain nor path to victory lies by doing so. even if Tony could slow Thanos somehow, he know it wouldn't matter. just like everyone else trying to do that failed ultimately. one and only thing to do after getting all the stones is just defeat Thanos. that's the Endgame. Tony just did that.

P.S: Comparing the knowledge Thanos had and Stark had on the stones would be a huge factor in the victory or defeat if Stark tried to control the stones. Thanos has a good knowledge about stones by learning about them for a long time, He surely knows more that Stark about manipulating stones, how they work and how to achieve his target efficiently. Tony Stark (being a genious), probably knew that he doesn't know to work stones like Thanos do. so his wise choice would be just go to the ultimatum.

Iron Man used the nano particles comprising his leg armour to form the Infinity Gauntlet on his hand, so there were not enough particles for him to make boosters to fly away. Also, when Doctor Strange looked at him, he understood that this was the only chance of 12 million.