It does feel a little weird that the battery outlasts the storage. Maybe I should've gone with 256 GB SD cards == 40k photos.

Does it start replacing the oldest photos?

Currently it replaces the oldest photos, yes. (There might be use cases where you only want to keep the first 20k photos though -- if anyone wants that, get in touch and I can add a setting.)

I figured that was the case. That makes the most sense for your original use-case. I ordered one last night, are you seeing a large uptick of orders from this post?

Definitely, the HN crowd has been kind.