I believe Greymore contains all the other expansions including Elswhere. (I already own Morrowind and Orsinium) Do I get anything extra from buying Elswhere now and then a month from now getting Greymoor?
Greymoor upgrade includes only greymoor, but when greymoor is out elsweyr becomes dlc, meaning you will be able to purchase it with crowns, and get it for free with eso+.
The Digital Upgrade versions do NOT come with Elsweyr. If you buy the Standard Edition you get the base game + all Chapters which is aimed at new players and not ones who already own the game. So, yes, if want Elsweyr buy it on sale. Also, check which version of Greymoor you bought...
Necromancer will probably be sold separately, so if you want that class it's probably more cost effective to get it now with the Elsweyr chapter than later for Crowns. For example warden class is sold separately to Morrowind dlc now and it costs 1500 Crowns.
"I bear the cruel weight of certainty. Total, absolute, relentless certainty. People rarely comprehend the luxury of doubt... the freedom that comes with indecision. I envy you."
―Sotha Sil
Yeah I'd recommend picking up Elsweyr if it's on sale, especially if you ever want to make a Necromancer. When I bought the Greymoor Collector's upgrade, it did NOT come with Elsweyr and had to buy it separately.