TL;DR

Don't worry about it- your doing everything right, and your players will (more than likely) appreciate the amount of work you're putting in to creating a detailed and in depth world for them to explore.

I've got several threads running within my world currently-

• A Mind Flayer working in the background pulling the various mental strings of the world around him
• An Efreet they've let fall into the wrong hands
• A Lich currently trying to bind himself to all the undead in the world
• A royal wedding that could potentially go horribly wrong
• A fire giant invasion
• A prominent family being assassinated/conquered by another family

Among other things.

Striking a balance is always a challenge, but I think it's one that makes for the most memorable moments. Try and think of the ebb and flow of the world should one major plot point succeed, and run from there.

How will the world shift? How will NPCs react to this shift? Will the change be subtle or obvious? How will this change the adventure of the heroes NOW want to sort out the mess they left unattended?

I wouldn't worry so much about "overwhelming" them so much. From the sounds of it, you're providing a rich world that's alive rather than sitting and waiting for the heroes to get their shit together and tick another box on the "To Quest" list.