I was doing a bit of research on flywheels as a capacitor, but I found during my research that large ships actually use flywheels to keep the ship from rocking.
They have flywheels made of carbon fiber nowdays that can spin at very high speeds. The main concern of flywheels is the amount of energy created in them and the explosive power if the material breaks up while spinning. In a seastead you could have the flywheel chamber surrounded by water…any explosion would be dampened by sending shrapnel into the water.
With vacuum sealing and magnets they’re very efficient.