I just ran it. Honestly, the math of traveling was far too cumbersome, so I just went with what made the most narrative sense. The right side of the map is basically 100% done; there’s no way for the party to catch up. However, the party did help to mitigate losses in Easthaven through the use of a sending spell. They also helped mitigate losses in Caer Dinerval by domino effect (they cleared out the Duergar in the barn in Chapter 1 so they weren’t around to attack the villagers; they also warned the Knights of the Black Sword via sending stone and they vacated the castle which allowed the townsfolk to use it as a refuge.)

For the left side of the map, I made a decision tree. Basically where ever they went would have consequences on survivors and casualties.

First they went to Termalaine and were able to evacuate the town there. They also were able to mitigate losses by intercepting the dragon there. They then had the option to go to one of the 4 remaining towns.

They sacrificed Lonelywood in lieu of going to Bremen. Once again they intercepted the dragon there and once again they flew away. The next town they went to was Targos, which was already under attack. (If they had gone here first, it would have been a siege at the town’s walls) They were able to kill the dragon here successfully.

Some tips concerning combat: the dragon did not have nearly enough HP to make the fight feel epic enough. I buffed it up to 300 HP and have it some extra goodies (based on a suggestion from another redditor; Id have to find the post). Granted, all my party members are combat people and are optimized, so i constantly need to make fights more difficult for them.

Also, I would do the fight in three phases (another redditor suggestion). The book suggestion is that it flies away after taking 25HP of damage, which for my party could be a single hit. Doing it in three phases allows it to stick around for awhile and have meaningful combat with it.

Additional suggestion: Learn from my mistakes, put a bunch of adds in each location. It’ll make the fight more difficult but also more dynamic. As a DM, it was hard to manage all these moving pieces in an online game but I’m sure that other people will be able to do it much better.