One obvious lore point, I think, is that if the Thargoids were in the Coalsack as early as 3111 - and have returned there now - the whole "hide behind the bubble" theory is looking even more shaky. It never made a lot of sense, but if the Thargoids have always been on both sides of the bubble, there's no practical way they could "hide behind it" even assuming that their hyperspace travel speed is glacially slow.
It also wouldn't surprise me if other nearby nebulae also have barnacles seeded, even if they're not "ripe" yet.
When you see that many nebulae are filled with guardian site , you can guess that nebulae could be the "door" for goid or for another form of travel.