How strong.
Is the mold strong enough to walk on?
Would it deform if you did that? Probably.
You can get pressures approaching 15PSI with the vacuum.
Work that out and I suspect your mold is deflecting.
As the finish is optical, even a small deflection will be easy to see.
Turn it over and cast about 3-4" of concrete, but it would be best to calculate how thick it really needs to be.
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.