I find it interesting that the hot stage jettison is after the boostback burn. That implies to me that the purpose of the jettison isn’t for weight reduction. I wonder if there are aerodynamic control reasons to jettison the hot stage.

I also wonder if the hot stage ring is expected to sustain enough damage to be a single use part anyway.

https://m.startribune.com/minnesota-man-builds-castle-with-3-d-concrete-printer/273410261/

This guy DIY’d it way back in the early days of concrete extrusion. He actually let a college class I was in see it. It was definitely an involved project and the result was pretty rough around the edges. His system was pretty slow and took a substantial number of batches to complete. Getting the machine and mix balanced for extruding all in one go would be pretty difficult and you’d probably need to have it mix continuously rather than in batches.

Unfortunately I don’t remember much about the type of mix he used or the pumping system. So I can’t guide you in the right direction.