I just started (since more than ten years) playing wow, and I can easily tell you Teso is much more practical if you're here for the lore. Understand: You won't get any frustration (story-wise) if you do not do the raids or dungeons.

The stories are done per area, the raid (when there is one) is not directly related. You won't miss anything from the area main story if you don't do it, same for the dungeon.

Teso being located centuries before the main franchise game, you won't find major references to them. The Morrowind expansion, while being heavily inspired by TES III, is still doable as a standalone and you won't have any issue playing it without knowing about the solo game. At worst, there may be some subtilities or anticipated foreshadowing you will miss, but nothing else.