There are two things to keep in mind:
- Dr. Strange knew the final battle in Endgame was going to happen since before the fight with Thanos on Titan, so he was moving the moment he got undusted.
- A lot of time passed between the Hulk's snap and the Captain America's solo stand.
To elaborate on the second point, after the Hulk's snap: Avenger Tower was blown to bits by Thanos' ship, Thanos told Nebula he would wait while she found the gauntlet, Hawkeye escapes from the demodogs with the gauntlet, Gamora and good Nebula kill bad Nebula, Thanos fights Cap/Thor/Iron Man for a while. Some of that overlaps, but it took maybe 30 minutes in movie time. I would put it at an hour or two in real time.
Regardless of how much time it actually took, let's think about it from Strange's perspective:
- Looks at futures on Titan.
- Sees the one future with the all the armies having a showdown on 2023 earth culminating in Tony's snap.
- Spending at least a few hours sitting around thinking about how to realize that future while they wait for Thanos.
- Gives Thanos the time stone, then has some more time to think about the 2023 fight (which for him is in just a few minutes).
- Tells Tony it was the only way and vanishes.
- Immediately wakes up, tells Peter is has been five years and starts spinning his sling ring.
- Has all the time outlined above to implement marshaling of the forces around the universe and organizing Wong and the other sorcerers to be their transportation.
To answer your question
how were they all able to get brought up to speed and ready to fight in the short amount of time between the Un-Snap and the "portals" sequence?
Dr. Strange had been working on the travelling salesman problem basically non-stop since he viewed the 14 million futures, and had all that time to optimize the preparation of forces during the (extended) time between the undusting and the big fight.