Why would a chain be a problem after 500 miles? Of any kind of problem short of outright abuse? Shoot, it would take 100 miles to do the initial riding on the OEM lube on the chain before reapplying or doing whatever you do after that.

That's something that stuck out to me here.

Next, every bike shop I've ever been to has this issue with scheduling service. Nobody can give you any accurate window to dropoff and pickup that is reasonable. It's hemming and hawing about how many bikes are in line but just leave it there indefinitely and we may call you someday. Somehow the vehicle repair shops get this right, bicycles get it wrong.

I think bike shops could resolve the "diagnostic" bit like car shops. Diag fee of $25 applied to repairs if you choose to do so. A basic bike is a freaking bike. Not much to it. For the basic bikes just have the prices and work be the prices and work on a menu. Pads, cable/housing, brake adjust front/rear, shift adjust front rear, etc........

If I was bike shop king for a day:
-you get a specific date/time to drop it off and pick it up
-you get an email during that period of the "menu" items to pay to do
-you choose the menu items, the $25 diag is applied to those items assuming they exceed $25
-you pay and pickup when we told you it would be ready

Doesn't sound hard, but must be.