This question is for those players on PC NA who're Steam users who bought all ESO content (up to Blackwood) from Steam in past. Currently, Steam isn't offering an upgrade bundle for the new Chapter. So the only way to buy High Isle on Steam atm is to buy the entire collection. Which means rebuying all previous Chapters from Morrowind to Blackwood.

Didn't want to waste money rebuying existing content. So as a Steam user, is it possible to upgrade the game directly from the menu once you login? i.e. from that upgrade option at the login menu or even after entering the game from the Store.

If this is possible, then how does that affect Steam players for future logins? Since the purchased Chapter wasn't directly bought/installed on Steam but from ZoS. As one of many Steam users, I had to endure absolute HELL in the past trying to login to the game. First being authenticated through the Steam client backend connection. And again through the ZoS server side connection. Spent ages with CS trying to resolve this login issue. So I'm concerned buying/upgrading the game from the game menu (presumably being purchased direct from ZoS/game Store and not Steam) will recreate this account login nightmare.

TL DR
If all your ESO game content was bought from Steam, but you try to buy the upgrade from ZoS? from the game menu (or the Store after you login to the game) will this create future game login issues?