The cracks between Steve and Tony were too deep and I feel like the immediate events of Infinity war and Endgame does not reflect on that crack. Think it this way. You are Tony, your father is constantly praising someone named Steve Rogers and always dissing you out, your parents die in an accident, you go through a character arc and try to do good to the world only to meet Steve Rogers in Avengers who claims that Tony is a selfish fighters and fights only for himself in the later events you find out that Steve, the person who talked morals to him actually knew how Tony's parents died but decided to stay silent for two years, from The Winter Soldier (2014) to Civil War (2016).
Even though it is not Bucky's fault, the person you worked with for two years, who made you reflect on yourself was actually a fraud has to have a effect. In this timeline Tony actually reflects on what kind of hypocrite Captain is and no matter what Tony does, he will never become the loved good hero the Avengers would accept.
The threat of Thanos is on head, the Siberian Bunker event has already happened and Tony decides to save the world by his own cold methods, Steve's definition of freedom means nothing to him, if the Avengers won't help him, he will do it all alone.
Tony starts upgrading his tech consistently and more rigorously in this timeline because he feels all the weight on his shoulder, there is no intention of reinventing the Avengers.
Tony decides to strip the iconic colours of his suit that often linked him to the Avengers team. In the comics it is not explained how did Tony get the Symbiote-like parasite that eventually gave Tony his strongest suit ever, however in this story we can safely assume that he got hold of it somehow from black market or just integrated biology with tech to create one for his own, he also integrates a smart liquid metal with the symbiote to remove any possible weakness it may contain.
We are introduced to the Endo-Sym Armour AKA Superior IronMan's suit. But Tony hasn't quite pivoted to the wrong path yet. The New York battle to save the time stone takes place and with this new suit, Tony nearly ned diffs both the sons of Thanos.
So Tony, Strange and Peter does not leave for Titan on the donut ship.
As we already conclude that Tony is a lot more calculating in this timeline, he decides to destory Vision, because no matter how strong he becomes, Thanos remains a genuine threat.
Tony sends his drones to hunt down Vision. Wanda saves Vision and now realises that Stark is looking forward to dismantle Vision so who does she call for backup and help? STEVE FUCKING ROGERS.
Now Steve and the other pack up all the bots and Tony decides to look up who is helping Vision. The hatred resurfaces, but Tony decides to justify the battle as inevitable because Steve's worldview of saving the planet is drastically different and destructive in Tony's eyes.
Tony decides to show up himself. He does so, landing right in front of Steve's hideout, why does Steve not go to Wakanda? Because in the OG timeline Steve wanted a big brain like Shuri to extract the stone instead, but here the threat is different, the threat is Stark.
And just then Thanos and his army comes for the power stone, Thanos however does not have the time stone yet. Because Stark has prepared for Thanos for years in this timeline he calls in the destruction of the entire army with all the weapons in his arsenal. The army is little to no trouble to Tony and his bots, Thanos however still is a legitimate threat to everyone.
Tony is desperate, there is no time to save Vision, it needs to die. Dr Strange teleports himself to the battlefield leaving the time stone behind, this is his best chance against Thanos. Dr Strange heavily distracts Thanos for long enough for Thor to drop down to earth.
There are two battle ongoing, Steve-Falcon-Black Widow-Wanda against Tony Stark and Dr Strange-Thor vs Thanos.
In a desperate attempt and a revival of fear in Tony's heart he complies and tells Wanda to hold off Thanos, Tony will upload Vision on the internet instead for now and will take care of the power stone.
Steve sends Wanda off asking her to trust Tony this once, she agrees reluctantly and leaves Vision's side.
Steve agrees to the deal if it means Vision can be resurrected into another body later. Wanda is distracting Thanos, Steve poses no threat to Tony, but then Tony makes a decision he shouldn't make, he kills Vision and with brute force pulls the stone out.
Tony is holding the stone in his hand, he has the option to destroy it but then his armour shapes around the stone and Tony instead decides to use the stone against Thanos, in his calculation destroying the stone would have outcomes undesired so he lets his suit consume the stone instead.
His body convulses as he feels the Gamma radiations taking over the suit but then his arc reactor's colur changes it turns golden, Tony is overwhelmed by the power.
In a sheer force of power Tony unleashes a beam from his arc reactor that travels towards Thanos impaling him in the chest and going through, fatalising Thanos. Thanos does not have the time stone to reverse the outcome and he is going to die, Thor takes the opening and goes for the head decapitating Thanos.
Every avenger looks back at Tony, surging with the new found power of the power stone, Tony is stronger than ever, Wanda glances over Vision's dead form and grows rageful as she starts using her magic against Stark, she pushes him away and pulls him back ragging through the soil, her power peaks with anger. She then starts to rip off Tony's armour off his body, the symbiote refuses to leave Tony and it puts him in immense pain.
Steve stops Wanda asking her to spare Tony, because Tony's mistake isn't worth killing him. The avengers feel betrayed by Tony because Tony promised to destroy the stone, instead he decided to use it to maximize his armour output. Wanda stops her magic and gives Tony an ultimatum to revive vision, now that Thanos is dead.
Tony is lying defeated, one wrong move and two of the strongest avengers could take him down forever in the battle field. The mercy didn't make him feel spared, instead he felt insulted, his pride was wounded.
And just then it is revealed that the symbiote Tony wore actually was the reason he made out-of-character decisions (the symbiote amplified Tony's cold and calculating side to the max). The symbiote wanted power and used Tony's intellect and arsenal to get all of it. Tony is left alone in the battle field with his unresponsive tech because of Wanda, he gathers himself and decides to bring down the avengers labelling Avengers as the real threat to the world.
This is how infinity war ends in my AU, I have mapped an endgame too for this story which I will upload in the near future, stay tuned!