As we feared and could not confirm without tangible evidence
The dreaded CleanMyMac aka BrickMyMac is installed and used
Any Third Party Applications that will interfere with the normal operation of the OS, is an invitation for disaster.
Certain Applications maybe available on the Apple Apps Store - this only means the Developer is prepared to pay Apple a percentage on each sale.
What the Application may do to the computer is up to the User to check this out before purchase
Disk Cleaner
CleanMyMac , aka “ BrickMyMac “
What the Application may do to the computer is up to the User to check this out before purchase
To put the Third Party Disk Cleaner / Optimizer in context and the damages it may have or has already done.
This type of applications can or will Muck Up your User Account ( Home Folder ) of this machine.
It does not touch the Operating System itself, unless you consider your User Account ( Home Folder ) as part of the Operating System.
The Operating System is hosed
Here, I have a small and important difference with a another posting
Always make a Time Machine Backup before proceeding
Emphasis on User Account Only
If going this route - I suggest Not using Startup Assist to migrate everything back.
This will probably Re-Introduce ( CleanMyMac , aka “ BrickMyMac “ ) back into the Operating System