The primo is excellent. MOST excellent. Add a plug to your z motor at the top of the z tube and just put together extra z assemblies, motor and all, with all the money you just saved on R&D.
And if you get around to mineral casting, make have a look at the
Stainless Steel - Quick and dirty flex test.
I haven’t tried filling my pipes with anything because the arguments haven’t been compelling enough to overcome my laziness, but it DOES come up an awful lot. I haven’t seen any actual tests though, just speculation. If you were to test with a similar protocol, you might be able to put this to bed for a lot of people whether it comes out better or not quite.