I believe some laser equipment does have the 20µm that you are looking for:
Displacement measurement sensors
This product for example has a 20µm precision, but it is said that the precision depends on the optical properties of the surface, so it would require a test for your solution.
OD Mini - Compact, lightweight sensor for precise measurement
Also, another set of options is to measure properties of the liquid that are proportional to the volume of the liquid (supposing that the only variable of your experiment is the height and that the liquid composition, temperature, pressure or any other variable doesn't change). In that case you could, for example, measure the liquid resistance from one side of the cylinder to the other. However this would require a custom apparatus.
Another option that falls into this category (of using properties proportional to volume) is to use absorbance, putting a light sensor at the bottom at the flask and a light source at the top (or the inverse). In this case the precision is only limited by the precision of your apparatus. However a precision measurement might require a sophisticated apparatus (I don't know how the 50µm precision translates into complexity in this case).
And, as it was already mentioned, mass is another option. Coupling a reservoir to the cylinder which can be measured by a scale. If your experiment support this modification, I believe it is the easiest option, since precision scales are available at most labs.
Hope my answer provides some inspiration and please keep us updated on the results!