although i have not built a spinner at all i could help answer a few things.

Shaft material. high grade Alu (7075) Or Titanium

Bearings are better (less rolling resistance = More RPMs = More KE) but more complex ( correct bearing, preloading etc)

Bushing cheaper and simpler but higher rolling resistance.

I would suggest reading 360 build log, Team Eventorizon build log. Really any spinner build log