This could work

[masterada]

Hey guys!
What about this sollution?
printersketch.jpg
If the size of the 2 containers is right, the resin should stay in place, while the inner container floats down (didnt do the math). The z-speed remains measurable the same way it was before.
The sketch misses some stuff (like some supporter rods or something for the inner container so it wont float randomly in the x-y plane, and a plug from the inner container to reset the setup).
The laser is in a fixed position above the whole setup.