Will Not Boot, Hangs at Apple Logo/ Mostly Loaded Progress bar.

The device originally became very, very slow— once logged into any user account it took forever to open software, complete basic commands, etc.

Ran disk utility in recovery and came back with a fsroot tree error.

Re- formatted the drive and reinstalled MacOS.

Device then would boot OK— no immediate issues seen. Device owner then picked up iMac.

Contacted by the owner one week later. She states it was OK for a short while (she says she used it for just a few hours). One evening owner shut down the iMac. Upon startup the next day, the device is stuck at the Apple logo. Progress bar loads to about 99% and hangs.

Attempted to boot- up myself, saw same results. Let computer hang at 99% progress bar, came back about 45 minutes later and was showing user account log in screen. I can see cursor blinking in password input field, but cursor was frozen. Switched out keyboard/ mouse combo to confirm, same issue.

Was able to boot into external drive running Mojave on said computer. No issue with that same original keyboard and mouse. MacOS on external works just fine.

Ran Apple diagnostic. Came back clear.

Decided to replace hard drive. Installed Mojave on internal drive from another computer before replacing. Tested HD would boot OK. After installation of new known- good bootable HD, computer still hangs at progress bar at Apple logo.

Ran Apple diagnostic. No issues found.

Attempted to boot from the old HD on my own personal MacBook  (which I originally assumed was corrupt) and I just get the prohibitory symbol. My Mac is running 10.14.6.

Decided to format and do a clean install of Mojave on new internal drive on client computer on last time. After install, was able to set up user account and load user account just fine.

Shut down the computer, attempted to reboot normally. Computer still hangs at progress bar in the same fashion.

My only thought at this point is the logic board. But I have no idea why I would get the prohibitory symbol with the original HD.

Edit: Upon a suggestion below, I wiped the drive and installed an earlier version of MacOS (El Cap) And now it’s working just fine. Rebooted several times to confirm, no issue. Is it possible that something happens to this computer everytime it tries to get on 10.14.6?