Now that I've gotten to what feels like end game I can give my full thoughts:
Uniqueness: This game has what I feel like is a new and unique "puzzle" section in the middle when it comes to maximizing Lunarity for the Rebirth upgrades. I see many games do the AntimatterDImensions "studies" thing where you have a whole array of upgrades and you can only buy so many and the puzzle is to figure out what the most efficient set is, which always feels like a puzzle that is too difficult to solve because there's just so many variables at this point.
In this game instead, the puzzle section happens earlier so there's less variables, and instead, it's about playing the game a little differently rather than solving for which set of available upgrades combos the best. How can you get to the 1e6 stress in a way that lets you still Learn enough times before dying. It's not obvious at first, but once I figured it out, it's a much more enjoyable "puzzle" than maximizing a set of complex upgrades.
Theme: I feel like the theme comes online far too late. It took a couple real life days to get there. I think you could introduce the concept of Solsperson/Lunesperson as soon as the Rebirth menu becomes available. It will add an interesting bit for the player - The Lunarity bonus to Solspeople will make them feel like the better option initially, then when you unlock Learning you'll see the benefit of Lunespeople and your priority shifts.
Fun: The "chance" to lose Influence upgrades is what made me stop. I went 5 rebirths in a row, buying a Settlement each time and immediately losing it. It made it feel impossible to build up to bigger numbers. It also means that I feel like I can't ever spend a life just farming Lunarity. I ALWAYS have to farm knowledge because I will lose the upgrade that lets me farm the Lunarity in the first place.