I'm just finishing Troubles in Otari. And doing the same thing. If you can take something from your prior game and slide it in even better.
Here's what I'm doing. In the beginners box, I added a ysoki cleric as an NPC (he was investigating a kobold incursion) bc I figured my group would need some healing to survive. Now that they're finishing up in TIO, they're going to run into that NPC, who is now leading his people in clearing the vaults. His forces are stretched and his advance has stopped at level 4. So we'll be starting in level 5, with him as a narrative guide as to what's happened so far.