And then there are people with heavy legs naturally - I take a deep breath, hold and after 10 seconds oir so, my feet are down and I start sinking; I am naturally negatively buoyant, so I feel your pain on this
as Spectrum mentioned, your buoyancy is like a see-saw, teeter-totter, etc - like me you need to get some positive buoyancy below your center of buoyancy to help keep your legs level - I have taken to keeping my knees bent, concentrating on keeping my feet up, and I use far more frog kick, or sculling than most
so after trying to adjust the tank, wearing non-ditchable weights up high and the other recopmmendations, if the feet still drop due to naturally higher body density, try keeping the knees bent and feet high