I only know about the SP training, and my information may be out of date. Several years ago I was asked by a dive shop/SP dealer to service their regs, and they offered to send me to the certification class, which was a Saturday/Sunday class in a hotel in Houston. Apparently, at least at that time, the sole qualification for getting certified as a SP tech was employment at a dealer. I thought about doing it but in the end I decided it wasn’t what I wanted to do.
DIYers looking to get into servicing their own gear are pretty much on their own when it comes to the major manufacturers. There are (or were) a few smaller companies that offered classes to divers for self service and would then sell parts to people that took the class. Maybe deep six still does that, someone on this forum will know.
Then there’s the ‘off the books’ online class offered by rsingler, a forum member who has excellent knowledge and experience fixing regulators, and he’s really put great work into this course. I’m sure it’s way more comprehensive and in depth than anything offered by the manufacturers. That’s probably the best bet for anyone interested in working on regulators.
Regulators are not that complicated; working on them is pretty straightforward if you have a good mechanical sense, some patience, and are willing to invest time and money into practicing. I find it rewarding and I love the fact that I keep a whole stable of regulators for technical diving in tip top shape for very little money. I also have the ability to fix pretty much anything that might go wrong on a dive trip. I can’t remember spending more than $100 on a regulator, and the performance of these rivals anything I’ve ever seen. There is a HUGE amount of hype and misleading information about scuba regulators, and I like the fact that I can easily see through it.