I trained an electrician from scratch on all makes
he eventually went to work at a VW agents (long story) and it was a pleasure to see him become so knowledgeable working there as their electrician. Working for me was a good grounding for him doing a huge variety of jobs
This job is becoming a nightmare in terms of the ammount of brain power you need to understand all systems on all makes
So I think to specialise is a good way to go so you can focus your available grey matter on one make
In the future if you went back to an independant you would be highly valued as the resident VW expert.
He didnt regret it and is better for being good at one make rather than just OK on all.
I try to be good on most and its only the collective knowledge we all have here that stops me from becoming an aromatherpist or some thing less stressfull
I know most of us HERE are professional but I see so much bad workmanship on my travels due to garages being unable to cope with all makes
Barry