My best calf raise was <300 lbs. I used a machine that rested on my shoulders and was afraid I’d have vertebral collapse if I lifted too much.
I congratulate and admire you!
Love doing calf raises. Oly bar on a squat rack. To get better stretch, a 2x4 or a couple of stacked 45s works good. Tried a step aerobics step, but none of them I’ve seen will withstand the combined weight.