Thank you for your kind words! I had more content planned in my mind (enemy types, character abilities, radio chatter) so it should've been more variable, but those didn't make it in. Actually, level length is not something I thought about at all when designing it, but now that you mentioned it I'll have to remember it, thanks.
The smoothness comes in many parts thanks to 2D physics interpolation. You have to explicitly enable it in project settings, and take care of what goes into `_process` vs `_physics_process`, and cancel smoothing on case-by-case basis when spawning/teleporting entities. It takes some non-zero effort to keep it bug-free, but the visual payoff is great, especially on high-frequency displays.