You make some good points. I can see it from both sides and the certified world offers some additional challenges compared to EAB. If it was EAB Garmin can give you an RMA and you can handle it yourself, in the certified world equip exchanges must go thru a dealer.
So from your statement I 'assume' none of the dealers you asked for quotes installed the G5?
This is where folks underestimate the value of supporting their local avionics shop/dealer, if your buddy AP and you install a G5 that your purchased from ACS for example, then when something goes wrong you want your local dealer to spend time on it and fix it for free, well you get the idea (I see a lot of this). However you may have purchased the airplane with the G5 already installed by another dealer across the country so I get that too.
Your local dealer is in a lose/lose, once he goes down the road of a 'simple' repair like replacing the G5 battery, now all of a sudden he 'owns' the job and has to get you a new G5 because the battery he just installed didn't work, now he's 100% working for free, so a 1 hour labor charge to get involved is not so bad really. And then maybe you start developing that relationship by spending $125?
When it comes to warranty repairs we generally all lose one way or another, just a fact, but.... if you have supported him in the past he's likely to jump thru hoops to keep you coming back. I have a customer who brings me his RV8 to work on, he also has an Evolution with G900X, guess what, I helped him with some G900 issues even though I spent hours working on it for free, why... because I considered him a 'good' customer.
If I install your Garmin panel it's not cheap, but now you will get 100% support from me, guaranteed, what's that worth?
That's just the way the world works.
PS: nothing takes 5 min, I know that's an old saying but the reality is after figuring out what needs to be done, doing the job and then doing the paperwork there is no such thing as a 5 min job
And there are not that many of us small avionics shops around anymore, can you guess why? (hint: it not because we are all getting rich and retiring), so supporting us isn't such a bad thing.
PPS: back to the original poster, if you think Garmin is tough to deal with try to find a BK dealer willing to work on your TT AP!