I have been following your perspective, and I am all in for what you have shared here. But for myself there are variables no one considers or will even be objective enough to consider and discuss in possibilities.
Example is simple arithmetic to start with. Either the arithmetic in the 1500s was very very different and flawed compared to modern arithmetic, or Ussher was really really bad at it. It does not compute as accurate even on it’s face or in the scripture it’s self. But to even question it is heresy and blasphemy.
Now here is a variable not considered in either camp... What if a true scientific physical change has happened along the way, maybe even several times when being applied to the length of a “day” for the earth and creator?
What if one day for God in Genesis was actually a billion years. And later the length of a year was confused because of mis-translation with months or days in the lives of the characters in the Bible who lived so long? Here is where science could support the possibility of an all mighty creator and the Bible it’s self. And at the same time give credit to the scientific discoveries.
But can we drop the firewall long enough to support these possibilities on an individual case by case that might support our own arguments? No, it would be heresy and blasphemy to even consider this. So we digress back into the superstitious all or none dark ages in faith rather than be objective and consider possibilities based on newer science.
We have locked ourselves into the ignorant dark ages rather than even consider any or all new discoveries at all. Christians are stepping on their own feet with this one.
Yeah, the calendar has changed. How much, how little, complicated to figure out. The Sumerian king list show the dynasties as sequential, and it is known from other sources (the ancient equivalent of joint communiques, for example) that the listed dynasties were not sequential per se. That doesn't mean there's a 4004 BC starting line though.
The reign lengths on the Sumerian king lists (lists, because there are several copies in different places) also are way out of wack, with lifetimes of millions or 100s of 1000s of years, sliding down to tens of 1000s, and so forth. Since (as Samuel Noah Kramer wrote) the Sumerian city states didn't develop and use a single number system, my best, uneducated guess is, the king lists were compiled from multiple sources from each of the listed cities, and the compiler used whatever local number system to interpret the other cities' information. Perhaps eventually a lexicon will emerge from the body of cuneiform texts and vindicate this view, hope so, and hope I live to see it. :^)
If that's not it, then one alternative -- that the celestial body or other phenomenon used to count time has changed from time to time.