If you liked Project Hail Mary, you'll probably love Seveneves. I'd read a bit of Stephenson's early work, and I'd heard a lot of good things about Seveneves.
The core idea behind the book is that the moon mysteriously shatters into 7 different pieces. The pieces have their combined same center of mass and general orbit, but they gradually begin to collide with one another. The entire book plays out that idea and sees it through.
I'd say the book itself is a mix of hard science fiction (lots of cool science-y/physics-y ideas) and political thriller.
What follows definitely contain spoilers. If you don't want to know about them, read the book first and then come back here!
Part I: The shattering of the moon
Once the moon shatters, the immediate threat to humanity becomes the impending 'hard rain', where random collisions between the pieces of the moon will eventually rain down upon the earth.
Saving humanity from that sort of fate is an interesting thought experiment.
Part I ends with the Hard Rain, where meteors rain down upon the 7b people left on earth. I found this section quite poignant, with members of the ISS/Arklets saying farewell to everyone they knew back on earth.
Part II: The hard rain
This part mostly just aggravated me. There's too much playing politics, backstabbing, and naivete on behalf of the scientific crew aboard the ISS. JBF is both incredibly devious and incredibly annoying. The humans feel very Hobbesian; living lives that are nasty, brutish, and short.
I can see why Stephenson included it, but I kept getting annoyed at everyone who was trying to do anything other than "save the human race".
Part II ends with the "council of the seven eves", where each remaining member of the crew (all female) is allowed to create offspring with slight genetic modifications.
Part III: That 5,000 years later part
I found myself spending the most time thinking about part III. Part of it because it seemed so outlandish and otherworldly. And part because, it felt like it was the most implausible.
Things I didn't really believe...
Taking a step outside myself, I guess I really do believe that modern humans are fundamentally good and reasonable, and that race-based divisions would largely disappear if humanity were reduced down to single digits of people. If anything, Part III felt like a good 'test' of that.
Despite all that, there were a lot of interesting thought experiments. How do you survive for 5,000 in an underground cave? How do you re-terraform a structure like the earth?
Despite all that, I'd definitely recommend Seveneves if you like science fiction with a heavier bent on technology and physics. It's a great story, and fascinating set of thought experiments.