I started at 55, weight is 220. Also with athletic background wich is not good help for start because body wanted to fight the boat ))
Never did one successful one footed start (the only I got from one footed is torn hamstring and out of game for one month).
I did one billion of attempts with 50/50 success before I made all things much simpler:
The only thing I think about is pulling the front knee to the chest and back heel to the bottom.
And that is.
I do not think neither about angle of the ski nor how dip the ski tip in the water.
Only trying to pull front knee to chest as strong as possible.
The rest goes authomatically.
Some time ago somebody clever on this forum told - " two footed start is exercise of body relaxation, the only power is concentrated in your fingers"
I do follow this advise and making now 9 out of 10.
Hope this help you
PS just make sure that sole of front foot is directed to the boat, at the same direction as rope, not aside.
If foot aside you fall inside ))
Sorry for language, I am not native speaker.