[Scruff Bucket]

No technical, archetectural or engineering expertise from me, but see if tenants are able to have, or already have an acoustic piano, as they can range from 250 to +315 kg in a footprint size that could be comparable to the footprint you described of your proposed fitness equipment. Less expensive, upright pianos tend to be lighter (but not necessarily), and even some of the mid-sized grand pianos are heavier than 500 kg, but they may be more expensive than the fitness equipment -- or your apartment!

Or you could consider tension/resistance equipment with a smaller amount of free weights for training. You may already know that intense machine training is a good way to isolate muscle groups, complementing with modest free weight training to develop coordination and overall strength.

Otherwise, you may need to develop gentle techniques to avoid repeatedly bouncing/dropping heavy weights on the floor in your apartment, which might have long term effects on the integrity of the concrete below you, even with floor mats -- not including any accidental dropping of any sized weights from a significant height!

Alternatively, obtaining a membership to a gym (if possible) will let you work out with heavy free weights and specialized heavy duty machines that may be more satisfactory for you!