Note: I am by no means an expert. 
Masking first! When masking while working with clear acrylics or resins I have had some excellent results with regular old white elmer’s glue. It generally leaves absolutely no marks, is cheap, and easy to apply/clean up. I haven’t ever used it on FLabs Clear so I would probably test it somewhere less important first. However, if your windows/ desired clear areas need a super crisp line, masking tapes specifically for this would help. I prefer Tamiya tape, which comes in all sorts of sizes and includes a line for curves. That could be rather difficult depending on the size of your prints. With tiny details, laying out a strip of tape onto a piece of cardboard and then exacto-blading out a shape as close as you can and applying it with tweezers is tedious but effective (or I suppose you could try some pre-cut stickers?).
As for opacity/lightfastness, if you are already liking Golden, they have a heavy line as well, but as the name suggests it is thicker. For opacity and lightfastness it usually comes down to pigment more than brand/carrier fluid. Golden (and many other manufacturers) has placed scales or other info on opacity on most products to give you an idea. In my experience, most modeling or miniature paints tend to be quite transparent unless completely unthinned. When doing whites, it tends to be easier to avoid a pure white and go with something a little more pigmented, especially cadmium based (so generally yellow to red).
I can’t point to a specific brand and tell you for sure it will be better, though. I tend to check things on a sample piece to test things out beforehand so that I don’t ruin the piece, but I end up doing it more often than not anyway. 
Ranted a bit, but hope it helps somewhat!