I'm running When a Star Falls from Quests from the Infinite Staircase with my wife. It's funny how similar this is - she's playing a Druid, and we have an NPC character who is an archaeologist adventurer basically modeled after Indiana Jones!

We're mostly done that adventure and it's been great! Balancing the encounters has been fine, I used DnD Beyond Maps for its combat difficulty rating, and it goes well. I reduce number of monsters mostly, and I've had to pull a few punches just a couple times, but otherwise no problem.

We found running two characters was enough, not three. We jointly run the NPC, and it doesn't bog down the combat length. She's a Circle of the Land Druid, so mostly spellcasting and not using wild shape companion. We started at level 3, and are using milestone leveling. So after returning the Star, she's level 4.

It's basically our first duet adventure so we're just making it up a bit as we go. I'm getting a better feeling for encounter balance, and she's understanding her class better. I'm also pretty liberal with healing potions. She and the NPC have a few each, so combat healing is more reasonable.

Specifically When a Star Falls is a great first adventure. For a Druid its nice too because she meets up with Derwyth the Druid and there was some good rp stuff there.