The difficulty you experience, understanding the "timeline" of heaven, is a common one and the problem is simple. Most people view God, creation, time and eternity from our own perspective rather than from His perspective. From an earthbound view rather than from an eternal view. We assume, because we dont know any better, that our "experience" is an appropriate basis for reality and we thus instinctively draw the conclusion that God must somehow fit into OUR understanding of how things work. We thus fail to consider that God is Eternal and exists OUTSIDE of creation. He is therefor not subject to, or in any way, limited by any of the things we assume are real or true.
The entire physical universe was at once created by God. It was set into existence with all of its elements, rules, natural laws, dimensions and cycles. The moment it was created marks the "beginning" of TIME. The moment it is all destroyed will mark the "end" of TIME. Our life experience is bound by the bookends of time. Everything we know and perceive has a beginning and an end. Eternity has neither beginning nor end. God is eternity. He is yesterday, today and forever, the same! He is neither big nor small and yet He is both and He is both at he same time. He of course fills the "heavens" and at the same time is fully manifest and observed in the manifest form of a single Man, the Man Christ Jesus. He could die and yet be alive forever more. He could be born a man and yet from eternity past have existed as God. He could be hit with fists and beaten with whips yet he can pass cleanly through walls , disappear through a crowd of people, surrender His Life S/ Spirit descend into hell and ascend at will from the grave unmarred and unscathed, to earth and then again, at will, translate to a realm beyond sight and imagination. All this to purchase and win the love of human lost and fallen human beings. This may all sound confusing and a bit difficult to accept or assimilate however this is exactly what the Bible tells us. His ways are higher than ours. Eye has not seen nor ear heard , nor has it entered into the heart of man the things which God has prepared for them that love him! In other words, you cannot imagine no matter how hard you try, how much heaven holds for you cor how it will be experienced or anything else about it beyond what has been "revealed." You will only be able to rest in your search for reason and understanding when you come to rest in the knowledge that God is beyond our reach and beyond being found out. That is why HE came to US. WE were LOST and He FOUND us. He came to reveal Himself and then He went away "to prepare a place" for you and He promised to return FOR you at some fixed point in time that you could spend ETERNITY in His presence. This is the whole message of the Bible. This is the GOOD NEWS .. the GOSPEL.. He loved us and sought us and found us. Our job is not to "figure him out " and fully "understand" him. .Its is to RESPOND to him with faith and hope and love as He enables us and to seek Him with all our strength, mind and heart. If you are simply looking for an eschatological answer, Jesus answered it. Heaven is within you. Small , contained, yet fully manifest, while at the same time OVERFLOWING ( for lack of a better term) the entire Universe and with no end to its power and reach ! Heaven entered the Time / Space creation and showed up in "material" form in the being of one Man , Jesus Christ. God with us. When we enter Him , we enter heaven.. When He enters us. Heaven is within you! and for all intents and purposes, at that moment, eternity has begun for you! Though "time" passes and disappears, eternity will never fade. The "dispensation timeline" taught by many prophecy teachers is applicable to earthly events and goes something like this. 1500 years from Adam to Noah ( from creation to the flood) Then approx 2500 years from the Flood and Noah to the Birth of the Messiah and now 2000 years since ( the time of the GENTILES or the CHURCH AGE. After this comes the 1000 year KINGDOM age or the Millennium. Then final judgement, sheep and goats etc and then the end of the old heaven and earth and the appearance of the new heaven and earth. ( note there are three heavens or 3 levels. ) this is not necessarily the heaven we think of as the 3rd heaven but more likely the first heaven given to replace the physical one that was cursed and destroyed. There are three divisions in this time line. they are the flood.. The cross and grave. and finally the 7 year tribulation. Although this is an inexhaustible study.. I hope this little bit helps..