I've tested some of the variables you've mentioned, but by no means all. One reason I haven’t done more testing is that when BCY-10 came out, I bought approximately a lifetime supply of it (bearing in mind that I’m 78). I'm sure that there is better stuff out there now, but it works fine, and I’m not inclined to leave a bunch of bowstring to my heirs.
I would think that the ideal string mass would be zero, but as that would be impossible to achieve, we should look for the lowest mass string that will deliver the characteristics that we want.
In my pursuit of this, I passed through the stage of building skinny strings, padded at the nocks and double served. I think that the actual mass of a padded, double served, skinny string is not much less than a string that has sufficient strands to protect the limb nocks and can be used with a single layer of serving. So with my BCY 10, I make all my strings now with 2 bundles of 7 strands, which gives adequate limb nock protection and arrow nock fit with one layer of serving, and is overbuilt for the 50# and under bows I now shoot.
Fortunately for me, when I bought the BCY 10, all bowstrings were about the same diameter so everyone knew what everyone else was talking about when they said a 14 strand string. Now, some bowstrings have significantly smaller diameter than was formerly the case, and more strands are needed to make the same diameter finished string.
Most bowstrings these days are overbuilt as far as breaking strength is concerned, which is comforting given that synthetic string material weakens over time, so may not be overbuilt if you are shooting the same string 2 years later. The rule of thumb is to have a breaking strength in the finished string at least 4X the load placed on it. One interesting fact is that the tension along the bowstring may be two or three times the force exerted by the archer. So for my 50# bow, the tension along the string may be 150#. 4 X 150# = 600#, which would be the minimum recommended breaking strength of the finished bowstring.
And that just about exhausts my knowledge of that!