It's extra content, pretty much a big DLC. I would recommend playing what you have now to see if you enjoy it before spending enough to buy another game on a game on a game you may not enjoy.
So I bought ESO a few years ago but never really played it and I've decided to start playing again. Is it worth me buying the Greymoor digital upgrade? I don't understand what these expansions do and if I need them now or is it only once I reach a certain level?
It's extra content, pretty much a big DLC. I would recommend playing what you have now to see if you enjoy it before spending enough to buy another game on a game on a game you may not enjoy.
That would be small DLC. Greymoore in my opinion is not large DLC. Large would be Wrothgore (sorry for spelling and if name is wrong.) Also Morrowind, Summerset and Elsewhyre were bigger and cheaper. I find Greymoore to be going downhill in the creative department for Zenimax. Paying more for less sadly.
I would say, no, don't buy Greymoore UNLESS you get it NOW so you can get the free house and pet that is offered to everyone who bought it.
That said, if you don't care for the house and pet, just get ESO Plus and get all the DLC (except "story" chapters) and you have LOTS of content. You even get Morrowind and I believe Summerset as well. So you will not be missing out on Greymoore UNLESS you want to scribe and "treasure hunt".
I bought it on sale, not worth the full price in my opinion. Save it for later, you have so much content still even without ESO Pluss.
I say get ESO plus just for the ability to use the Blade of Woe. For me, that is not killing people it's "knocking them out". Since they come back a few minutes later anyways. So nice to be able to use if you like sneaking and not just straight fighting all the time and role playing.
Good luck in your choice.