Lots of good but semi-conflicting advice about what level is appropriate. Thanks, glad for the input. Some folks have said not to start out too low, and one or two here and elsewhere have said to be careful about scaling--that the party will rise in level quicker than what a 1e module anticipates. I've been tracking XP all along but have been graduating toward levels awarded for milestones. The latter just feels more natural to me.
I checked out X4 and X5 last night, but was semi put off by the desert environment. Desert isn't what I'd anticipated doing. Still, a couple reviewers noted that X4 and X5 when run together made for a logically consistent compelling story, and I found that attractive. I could maybe reskin the environment to arid mountains rather than true desert, and that might work.
Re: X1, I'm thinking Isle of Dread probably needs to wait until it can be run alone as its own thing. I've put it to the side for now; it doesn't seem appropriate as a follow-on adventure for this group.
Castle Amber (horrible name for a really cool adventure concept) might be worth a look. I'd probably want to layer on the horror if I ran it, but that could be fun. I'm kind of into horror as a trope right now. My sense is that Castle Amber would be far more fun than I6 Ravenloft. Less about an iconic character (Strahd) and more about the party. I'd always rather feature the party.
I'm looking more for a sandbox than a railroad. I want players to have meaningful choices. One thing I'm thinking about doing along that line is planting adventure seeds in Stonefast that lead to pre-selected 1e modules. For example, I've distributed an old tome as loot. That could be a seed that, if followed, leads to the party doing S2.
Lost Caverns of Tsojcanth might have to be my choice. I'm having trouble locating a legal .pdf of it to purchase, however. Sadly, drivethrurpg says they
no longer sell it. Figures!