To make a long story short, I offhandedly mentioned a campaign idea for a bard dad getting his kids together for a road trip along the Sword Coast and subsequently had three people approach asking me to run it! I'm working through panning out a few sessions, but given this is the first campaign I will ever run I figured that getting some feedback for my Session 1 plans would be a good idea.

I plan on having a short session zero for everyone to create their characters, talk over expectations and house rules, etc. For the first session proper I was going to ask each player to describe a bit of their daily lives and interject with their father giving them an invitation to meet him in the Upper City of Baldur's Gate. He then pitches the road trip idea, but mentions he's a bit light on coin at the moment. Fortunately, he found an abandoned cave system that he's pretty sure has a small treasure hoard somewhere in there! He then pitches exploring the cave as a way to get to know each other a bit before committing to the rather large trip he planned to Neverwinter and back. I was expecting that with the individual character moments traveling to the tavern and chatter with the NPCs in the tavern that we would likely take about an hour or two, especially with players who like to roleplay a fair bit. Do you all think this is a fairly reasonable expectation? Should I prepare some other specific things in the tavern (maybe a bar brawl or some impromptu gambling) to stretch things out?

From there, I was going to give the PCs a meeting time and place, and a free day to shop for supplies and prepare for spelunking. I was thinking the cave itself would basically end up being relatively close to the goblin cave from the Lost Mines of Phandelver campaign module with some strategic monster and puzzle substitutions. I was going to drop hints that this was a minor treasure drop for Balduran for geographic appropriateness, with a nautical puzzle thrown in there as a safe guard and a harpy guarding the treasure. I am just scared of TPKing my players session 1 and never getting another session, but by the same token I know there has to be a decent amount of risk here. Would one harpy be a decent final encounter like the CR guidelines suggest? Would it be reasonable to have ankle-deep water concealing some light traps in the room, so the harpy would be luring the players to step into them with their Luring Song? And should I just be scrapping absolutely everything?