And speaking of controversial, if you are comparing dulcimers to guitars, it helps to use proper luthier terms. Technically, very few dulcimers have a "bridge". A bridge bridges the internal bracing. As most dulcimers either lack internal bracing or lack anything that spans them, most dulcimers do not have a true bridge. What dulcimer builder call a bridge, every other luthier calls a saddle. However, as to part of your question.
Most guitars have the strings anchored to the soundboard. Some dulcimers, Gallier in particular, anchor their strings in the same way. The vast majority of dulcimers anchor their strings to the edge of the soundbox. Comparing the string angle on a instrument with the strings anchored to the soundboard to one with the strings anchored to the soundbox is an apple to oranges comparison that will get you nowhere.
Jazz guitars anchor their strings to the edge of the sound box. I read an article in American Lutherie in which the author tried various string angles to see which was best. The author decided that a more shallow string angle led to a louder more jangly sound. As the angle sharpened, the sound became clearer. Beyond 15 degrees, the angle noticeably reduced the volume. The author concluded that 15 degrees was the optimal angle.
As noted above guitars are not dulcimers and dulcimers are not guitars. I build my instruments with the 15 degree angle at the saddle (bridge). I like the sound, but sound is musician's choice.
For the headstock, I also use the 15 degree angle. I have no particular reason for doing so other than it works for me.
And just for full disclosure, my dulcimers do use internal bracing, a true bridge and a floating fretboard. So take anything I say with a grain of salt. The attached picture is of a build I intend to finish this weekend.