Amazing post-mortem. The rewind mechanic was super creative and joining all of the game state into a SSOT kinda reminds me of the bullet hell pattern system I've implemented, which all the bullet data belongs to the shooter and not on individual bullets, so its easy to keep track on them, as such when the enemy gets killed and then all bullets are queued for destruction. I could see doing an implementation of a rewind for my game as well, but unfortunately not all the game logic is in a SSOT, which just wouldn't work.
I'm actually really impressed by the good amount of technical information in this postmortem, there are many things I didn't even know before. (I guess this is the problem of a Unity programmer that has all of these fancy tools but no deep understanding of low level programming...)
Good work, and you even participated into Ludum Dare afterwards! One thing that I've noticed that most jam games (myself included) unfortunately fall into, are balancing issues at release. You can do the most beautiful or well designed game, but in the end... There's always something that could be made easier, or more challenging. Or maybe you haven't noticed a detail that could potentially scare away some players, it's really tough. You managed to pull off really well surprisingly, and most things you've noted weren't even noticeable, audio seems really just fine to me (which by the way, the rewind effect just sounds amazing).
Looking forward for a more polished version of this game, really wish for an interesting boss!