So at a retail Apple location (which unfortunately doubles as their service centers), their solution was to try and sell you something new? Colour me not surprised.

They do have more sophisticated diagnostics than we generally have access to though. When I first bought my MacBook (also in 2015), it had a problem where it was losing data. I asked around my job and no one had heard of anything. I took it to the Apple Store (it was still under warranty, and I had an extended warranty to boot), and they couldn't "find anything" either. It wasn't until I was about to give up and walk out that he mentioned that the only thing their diagnostics software observed with something with going into suspend mode. I put two-and-two together and realized that by my simply closing my case when I wanted to do something else, it wasn't properly saving state. I made the connection and the store employee gave me a replacement laptop on the spot and I never had that particular problem again.

If you are ever in need of hardware repairs and you don't already have a reputable non-Apple repair shop that you deal with, you should consider checking out the Rossmann Repair Group out of NYC. Louis Rossmann has done lots of Youtube videos on him fixing problems that Apple had previously told customers was "unrepairable", and is in general a right-to-repair champion.