A flywheel in a vacuum floating on a magnet also just hangs there. But indeed the weight solution seems more simple.

You would need to supply a lot of power just to maintain a vacuum and levitating your flywheel. Not sure this solution would be very effective. Maybe with superconductors, but then cooling is your problem instead.

Or use normal magnets (non-electro magnets) The vaccuum, sure, this may take some energy depending on the insulation.