Isocyanates can be toxic because they are quite reactive. (The horrific accident at Bhopal involved an isocyanate.) They react with alcohols to make urethanes -- polyurethane means a molecule with many urethane linkages made in that way. The reason to have the isocyanate itself present is to make such linkages (and others) after the finish has spread, allowing it to be converted to a structure that no longer will dissolve, and that can be stronger. In this use it is generally a crosslinker.
"Silane ester" is a functional group more frequently found in things like bathtub caulk. In this form it is stored protected from water, but on exposure to water or humid air it reacts to ultimately form silicones, that is substances with silicon atoms linked by oxygen, and bearing carbon substituents. (Silicon is the element, silicone is the structure. They are different as carbon is from wood, though I usually try hard to not point that out.) Acetic acid can form as a side product during cure, which is why some of these smell like vinegar as they cure. While silicones are usually soft and rubbery, these linkages can be used to modify polymers in a way that hard polymers could result, with crosslinkers of lower toxicity than isocyanates. I would still not want to breathe them.
I understand you want to spray, and with appropriate ventilation these may be fine. I have used System Three WR-LPU, a water based clear varnish that is very durable. (I assume the crosslinker is a diisocyanate.) I used it on canoes so my standards for a smooth finish may not meet your requirements, but found that when applied in the hot of a summer day it dried quickly. But applied at dusk on a humid summer day it self-leveled to give a beautifully smooth surface even after brushing or rolling on: if you control temperature and humidity, you may not need to spray. It's the most durable marine varnish I've used.