GET THRASHED: THE STORY OF THRASH METAL (2006)

This is pretty typical of a metal documentary: totally nerdy and relentlessly complete. Metal people really know their history and love to dig into minutiae. This is well structured: gives a layout of the scene where thrash came from, talks about the SF and LA bands, then compares them to the NY area bands. The film argues that the west coast bands were influenced by the NWOBHM, but that the NY area bands listened to more hardcore and brought that into metal.

The basic format was to discuss each band with a little detail, while working them into the bigger picture. It covered CA and NY, crossover, Euro metal, other countries, etc. It didn't stop in the early 90s either. Mentioned Pantera as the only big thrash metal band in the 90s. Talked about grunge and nu metal and rap metal a bit too.

The funniest part was how sore the thrash dudes were about grunge. "We took these bands out on their early tours and then they ignore us when they got big." Lots of crying. People always talk about grunge killing hair metal, but I guess it killed thrash at the time as well.

This is worth your time. The Inside Metal series is obviously more detailed about LA (six movies worth, two specifically about thrash,) but this really covers everything you need.