I would appreciate if the fluid request threshold and item request threshold signals were separated. I often use mixed (fluid and item) stations, which goes funky when the request thresholds get weird. Or have a checkbox to "only transmit full trains" where trains will only be sent when all wagons can be filled to 100% and emptied entirely upon arrival.

You want to use the "station control" combinator, which is just a constant combinator wired to a cybersyn combinator which itself is set to station control. Set the fluid request threshold to whatever you want it to be in the constant combinator. It looks like this: https://ibb.co/KxNZnHs

The Station Control function of the combinator allows you to set per-item (and fluid) request thresholds, and mixed item stations like yours and mine up there is where it shines.