Closest independent shop to you that I have experience with is OC Tuning in Placentia. It's a second shop to AU Tuning in Huntington Beach. Raz is really great guy, and they are both inexpensive, and extremely knowledgeable. I believe closest dealer to you would be Cardinale, but I don't have any experience with them. I'm sure someone can chime in. VW of Garden Grove typically dgaf about mods and stuff for service, etc.
You can also search for the recommended Independent shop thread on here. There will prob be some in your area.
do NOT go to new century volkswagen (took 6 weeks to replace transmission and haldex, with 1 week for shipping the actual parts. 4 delays due to parts they forgot to order, and broke my axle changing the haldex somehow). Pasadena VW seems hit or miss also since they apparently have golf R specialists, but the guy I got straight up told me he was from mercedes and didn't know what he was looking for, when diagnosing the haldex. Spent a couple hundred ubering over to show him the issues myself, and once to just tell him the car was AWD rather than FWD.