My limited experience: You can buy ESO on Steam and link your Steam account to an existing ESO account. The account is what holds all previous progress and purchases, and I do not think that is reset when linking happens. Purchases against the account will have to go through Steam from that point on, though. You do not need to launch ESO and log in to the account from Steam, but obviously, you can if you want.
https://help.elderscrollsonline.com/#en/answer/36520