Trump's withdrawal was conditional not temporal which is why he kept blowing through "deadlines". The Taliban would attack something, Trump would stop the withdrawal and start bombing them until they stopped and then would resume the withdrawal.
Trump had clear objectives that Biden eschewed like getting all civilians, contractors and diplomats out before the military and removing as much military equipment as practical.
Bagram would certainly have been the withdrawal departure point. There was no illusion in the Trump administration that a western puppet government would persist. They knew Afghanistan would rapidly revert to Taliban rule.
This happened out of sheer incompetence. Joe Biden and his crew were not up to the decisions necessary for an orderly withdrawal. They are in way over their heads and simply overruled the military because the military doesn't currently possess the leadership needed to be forcefully persuasive with the President.
Everything here is speculative. This write up is this guy being an armchair quarterback with 20/20 hindsight.
The fact of the matter is Trump would have pushed through the evacuation better than Biden by blaming the military and intelligence and shunning any responsibility - as is his modus operandi.
Trump was a master of taking credit and passing the buck. Some may recall Trump had a failed military mission of his own:
https://www.nbcnews.com/news/us-news/how-trump-team-s-first-military-raid-went-wrong-n806246