Create a new world, an open that world and put it in datapack folder, relaunch game. OR if you use modded pack you can use a mod called openloader to load datapacks before worlds are even created. Auto applies it to any made world

idk, don't really have anyone to test it out with and if not. Im not 100% sure how I would set that up. :D idk much about coding etc I just learned how to make these data packs most recently

I mean, just load the mods/datapacks you want the advancements for then go into game, give all advancements then go into world save and every completable advancement should be in there

Achievements Expand Border\data\awbepd\functions\player, Then change ALL the run worldborder add 27 to , run worldborder add 1.
If you are running enough achievements to get to at least 2000 blocks then go for it. But I don't recommend changing it. 27 blocks is how many it takes completing all normal minecraft achievements and still being able to get to strong hold to fight dragon