I used Time Machine to back up my existing drive, and restored it after the swap, that part was very easy. I think the 1 TB SSD cost me < $100 on a black friday sale (I think it was in 2019), and I paid another $40 or $50 to swap it out. I would highly recommend going this route. I swapped it out for a 1TB SSD (Samsung) and my system is like new now. I have an older Mac Mini with a 1 TB HDD, same DRAM config as you (and was practically unusable). If so, that is almost certainly the cause of the spinning wheel. I am guessing Hard Drive, from the fact that it is a 2015. I found it interesting to see all the Unix type commands used outside of the normal user shell, which most of us think is the Apple OS. I believe the cost replace the hard disk was about $180 and took 3 days. I changed Gmail to forward to his prime email account instead of Apple Mail retrieving them, which solved the archiving problem. I disabled the Gmail account on the I-MAC and found the time machine recovery of that account had restored all of the Gmail messages just fine. He had always had issues with the Gmail account sending messages to archive folders. The I-cloud re-sync of Email had issues with with a GMAIL account saying it had 32 million message to re-sync that would take days to perform. The time machine restore was lengthy, but all restored fine. They installed a new disk and everything has been fine since. Although the disk diagnostics had found no errors, the tech said only a defective disk would fail at reformatting. They decided to try a disk reformat and reload, where the computer hung during the reformat. I took it into a Genius Bar where they re-ran the same tests, where reported the issues could be clearly seen, that things were taking way to long to process. Apple remote support ran many tests including disk checking, but all diagnostics came back clean. Additionally, re-boots could take 15-20 minutes. Any ideas on getting rid of spinning wheel.

Spinning wheel in os x utilities