Any updates?
Was the suggestion to take it in for repairs done by Apple Support, or at the store? Diagnostics by phone can only recommend so much (along with depending on the experience of whom you're speaking to).
Take the machine in to Apple or an authorized service provider for diagnostics. I get your frustration, but unfortunately components fail after 6 years— more so nowadays where quality control isn’t what it was 20 years ago, regardless of the supposed premium product. As mentioned, it really could be a number of issues. These things happen: I’ve had every model of iMac starting from 1999, and except for the 2002 lampshade, every single one has had some kind of hardware failure. My 2019 iMac was supposedly fine, then one day it wouldn’t boot after a routine Restart. The good people on this board gave an educated diagnosis of dead drive. Took it in and it was (apparently) a dead drive and bad RAM. Had both replaced (faster drive and more RAM) and the machine is back to its usual reliable workhorse-self again (fingers cross).