How to measure salinity?
Conductivity, it appears, is not the best way to measure salinity, there are questions of which ion, etc.

Instead, measure specific gravity. I get about Sg of 1.02 for your 3500 ppm.

Here is a commercial unit. It looks like they have the classic floating bobber sg meter, with a little magnet in it to read the position. I suppose you could make a custom bobber by filling a glass ball with just the right solution, probably just di water, and connect a sensitive gram weight scale to it.

The whole thing would need a pretty good 'still well' where water currents do not push your probe around.