If you want. Those, like Enderal, are total conversion mods that aim to remaster Morrowind and Oblivion in the skyrim engine.
Beyond Skyrim often gets conflated with those two, but its not a game remake. Its basically opening up the provinces AS THEY ARE during the events of the 4th era while Skyrim's plot is going on. Each province will basically have its own independent main quest, faction quests, and about as many side quests as skyrim had. Though Iliac Bay has to share world space with three provinces, since those three provinces are so politically entangled that it means they will be crossing over more.
Honestly, Bruma is the only glimpse into what they are promising so far, but damn, if its all as high quality as that, then the wait is worth it.
I don't really follow Skyblivion or Skywind, but I think their teams do share assets where it makes sense to.
Actually regarding SE vs Legendary, some of the BS Cyrodiil people talk about why they made the switch pretty early on in this stream.
If I were you I'd go for Skyrim Special Edition, it's much more stable and the modding community has had plenty of time to produce great content for it. The only reason you'd go to regular Skyrim for modding is if you want to install adult mods or a specific mod that refuses to move to Special Edition.