If you've been going a few years and you've been happy with the work, you may be better off just sucking it up. It's a lot worse when you have someone do terrible work and terribly overcharge you on everything. There are loads of shops that will do a bad job, not fix what they mess up, and overcharge you too.
There are plenty of shops that would charge every hour of work and then some. I've had shops round up every minute of work to like a quarter hour minimum. They end up doing 10x under 5 minute tasks that don't even add up to an hour but then bill you 2.5 hours by rounding it all up to the quarter hour minimum.
There's no shop that won't mess something up doing maintenance. The difference is how they handle it, how they bill it, and how frequently they mess up. Definitely avoid the shops that are messing things up all the time and don't make things right. Sounds like you actually got a pretty good deal financially and a discount. Not an expensive annual to begin with.
Going to a new shop is a risk (may be better, may be worse) and you're guaranteed to get hit with a huge bill the first year when they insist on doing everything different than the last one. You're in for a real treat if you walk away and look for another shop. Could get lucky but odds are you'll get screwed by a couple other shops before you find a decent one or end up going back and realizing you had it pretty good.