How do I justify giving such advice?
First-hand, personal experience. Now granted - it was years ago, but after being burned a few times, I started buying the Helm Manuals, my own set of tools, and I’ve never looked back.
But I’ve continued to price repairs (to see if I think it’s worth paying someone to do the work instead of me), and the cost difference (between Independent and Dealer) is *not* “negligible” ... it’s “considerable”. And for what (?) - to do rudimentary stuff like fluid changes, plugs, and brakes? To me, you buy the car from the dealer because you *have* to, but then once the warranty period expires you get as far away from that place as possible!
A little background so you know where I'm coming from I've been working on my own and other folks cars for 50+ years. Have built drag cars and drag bikes. Heck, back in the 70's I was part owner of a repair facility.
Still do my own work on my 35 year old Corvette but won't own a daily driver that is not under warranty. Way too many gizmos that can go boom in the night.
I used to always recommend your advice of finding a good, trustworthy local shop. Still do for routine maintenance items.
The bane of many vehicles these days is electrical issues. An owner will be best served by having a dealer tech resolve those issue. In many cases it is a must as your local shop doesn't have access to the dealer's computer system and related software.
My son is a service advisor for Car Max. They routinely transport cars to dealers for repair because they are the ones with the needed tools and the factory trained techs.
The biggest reason repair costs can be substantially different between independent and dealer shops is the labor rate. Can be as much as $50 an hour. But with modern vehicles, the owner may be best served by having the vehicle repaired at the dealer, especially if the problem calls for some troubleshooting.