Good luck! To my knowledge, Royal Apothecary Society is still active, so they could be worth checking out. Shadows of Lordaeron is another great pro-Sylvanas guild. I run Lordaeron Unbound, which is an anti-Sylvanas oceanic guild so that can be tricky with timezones and loyalties Though we’re active at our weird hours. There’s also a new Forsaken guild that’s popped up recently. I’ll find a link and edit it in shortly.
Keep an eye out though! May I suggest joining the Undercity United discord, if you haven’t already? You might be able to ask a more undead-focused audience there, see who’s still very active!
https://discord.gg/GndnJH
Edit: As promised, here’s the other guild whose name I couldn’t remember! Night Lords! They’re new, and should be active I’d imagine!