I've heard that Bronica was a Japanese adaptation of the term "Brownie," referring to the film size (120 film being "brownie film"); incidentally, Brownie cameras were named after characters popularized by Palmer Cox in the late 19th-early 20th century.
Apparently GE acquired the Mazda name rather than coming up with it in a marketing campaign. Interestingly the Wiki article also has the religious reference.
You could be correct that it means "medium" instead of Mazda, but I researched it about as extensively as one can in a "casual armchair internet fashion" a few years ago. I admit that won't pass academic muster. But I was looking at shutter mechanism information, not light bulb information. Once I found the Mazda connection I had to track down the Persian god reference by myself. But who knows if the Mazda connection I found was right.
(I know it must be true because I found it on the internet!!!)
The "M" was supposedly indicative of flash bulbs with the Mazda style base, what has now become known as a medium style screw base, while the "X" was indicative of the Xenon strobe. I cannot say what shutter manufacturer was responsible for the markings originally.