You’ll never be able to prepare for all the things your players might ask an NPC. They could latch onto any random NPC, including ones that were meant to be just background, and decide they’re a main character now. You’re going to have to make stuff up on the spot, and you should lean into it. Lots of favorite NPCs are made that way.
It sounds like you’re doing a great job. DMing can be a lot of work and stress, but it’s worth it. Just remember that your fun is just as important as that of everyone else at the table.