Great Conclusion to a vast Marvel Universe Avengers Series (SPOILERS)

Rating: 8/10 (28 April 2019)

While this a Marvelous conclusion to The Avengers series, but it has Huge Time Paradoxical problem and I may have missed something, if I am wrong correct me;
Just think how can Captain America fight his past self without knowing it already had have happened and he should been ready for it, because it already happened in his past.
Also how can Thanos of 2014 die or vanish from the Snap from Ironman because if Thanos of 2014 already vanished how can he then come back to use the Infinity Stone in 2018 in the first place, because then the original Snap happens in 2018 because he doesn't exists anymore, this is a HUGE Paradox.
And how can Steve Rogers stay in the Past with Peggy because there was no mention in the past movies that he lived with his girlfriend or gotten married to her, because as we saw in Winter Soldier that wasn't the case when Steve visited his dying girlfriend and when she died at Civil War there was no sign of Older Steve in her funeral service.
The movie should have happened like this way;
After getting to know what Thanos did with the Infinity Stones that is reduced them to Atomic level and when Scott Lang came back from Quantum Realm they should have put their knowledge together to retrieve the Stones from atomic level and use it then and there that would have brought back everyone those who have vanished in the Infinity War Snap and in the process Scarlett Johansson wouldn't have died :(.
After learning what they did, Thanos would come to take the Stones again to use it again but this time to wipe out all and thus the War begins and in the process dies Ironman.
Overall the movie was great nice surprises especially Captain America conjuring lightings not the part he worthy of using the Hammer, but the part that made me tear up is that when Tony is about passaway and Potts assures him that they will be Ok and nothing to worry about it, meaning go in Peace nothing to worry about you have done enough for this Universe and Rest in Peace :_(, damnit Steve should have let him build the layer of protection around the World that he wanted in the first place as in Age of Ultron, maybe the first version wasn't perfect but he had Jarvis, Bruce, and Vision then to assist him for god sake the man invented Time Travel!!