Rate of ascent/descent in both Position and Altitude Modes

Hi Folks,
I have been working on a large prototype drone using the PX4 FC and I am having trouble with the take off and landing flight modes. In both Altitude Mode and Position Modes the big problem is that nothing happens in the take off mode until the throttle gets up tp 62.5% and then then a fast ascent rate and then when you try to land there is very little reaction to downward throttle input, which makes the drone very unsafe to fly.
In our case this porotype drone design is to be used as a heavy lift drone (1000 Lbs plus) so control is very critical to its use!
Any thoughts on a way to solve this problem would great!