Very cool project!
I completed my turntable around 7 years ago & have been tweaking it since. My platter is also 4" solid aluminum but 12" dia. Plinth is from the same bar stock & 2" thick. The flywheel was made from 3.5" aluminum bar stock. The motor housing is also aluminum & holds a Hurst 300 RPM AC synchronous motor. The tonearm pedestal (aluminum & Corian) is adjustable. Everything is powder coated red &/or black. The platter is topped with 1/4 Delrin that is solidly bolted to it. The mat is Slaws "MyMatt'". Flywheel, motor, & arm pedestal are not attached to the plinth. The bearing(s) are modified aerospace valves with the "X" &"Y" portions separated from each other. They are made out of aluminum with a thin layer of hard coat applied to the surfaces that make contact with each other. The plinth & motor sit on springs while the tonearm pedestal & flywheel sit on Isoacoustics feet. It's been a fun project.
Please keep us updated on your progress.