That sad part is, this is not exclusively a technical issue.... this is literally something they brought on themselves with BAD design!!!
Why on earth would you gate the entire expansion beyond quests that eeeeeeevryone must do, that eeeeeeeeeeveryone is doing the moment they access the new area, that eeeeeeeveryone will obviously do at the same time, So early on upon reaching the new area, when your servers cannot handle the rush of players at the same time in a single player instance????????\

WHY would you have the solo instances SO early without content to filter people out along the way?

They should have had such solo MSQ instances AFTER everyone opened both new regions, unlocked the ability to search for all air currents, access to primal battles, access to all gathering nodes, and such. THEN there would be no bottle neck. Because people would have been spread out! This is completely a self inflicted wound, because they designed such a thing BEFORE thousands of players could spread out...