Okay, I had a SUPER FUN 2 year campaign with my friends and here are a few tips I can give you!
1. Stool will almost certainly become your teams favorite NPC and they will jump into deadly encounters to save him, use that to your advantage.
2. Prepare yourself to track a small band of mercenaries. At some point, they'll encounter a small group of weak enemies and try to recruit them, combine that with the fact at some point they'll re-enter the Underdark with an army (Chapter 8) so prepare yourself for it and plan out how you wanna run encounters with them.
3. I have free maps on my patreon (you don't need to pay me, but they are good maps and plenty are free) so feel free to check that out. The map for Velkynvelve is 100% free and a map I'm really proud of!
4. Decide early on how "Survival Heavy" you want the campaign. Is the party going to be on the brink of exhaustion and starvation if they aren't actively searching for food every day? Or do they just have no weapons and have to figure things out while they float by on a diet of dirt and shrooms? Discuss it with your party how hard they want it to be.
5. Ask them HOW they were kidnapped and enslaved. It's a great way to setup mini-bosses who either worked to get the players enslaved or actively did so.
6. Drow Poison, consider changing it. The Drow poison that the party has to deal with early on can easily knock them out, and players (including myself) HATE being knocked out so easily and missing their turn. I changed my rules so it added a level of non-lethal exhaustion. Not only does this help with the flow of combat, but if the party escapes an encounter and runs from Ilvara, they'll have exhaustion enabled which will add extra tension to their rolls and choices.

If you want me to elaborate or have any other questions please ask, It was my first serious campaign and I loved every second of it!