Apple Users

I was running old versions of MacOS. So old that some software I really want did not run.

The history of this is that I was on a cellular hot-spot, so upgrades were over quota.

I got the DSL (yeah, that's the best we have here) so I upgraded two macs to Mojave (which is still behind the curve, but this gear is getting old) I found an installer download so I didn't have to download twice, and risk the install failing due to an internet glitch (and having to do it all over, and over)

I didn't have any crazy problems to fix, but on the imac, the app store would just eat up all my resources and do nothing, and iMessage never would sign in no matter what I did. I had to sneaker-net apps from another machine and then verify them the first time they ran. Not good.

Things are better now.

I *did* a full backup of both systems before running the installer. I used SuperDuper with an external disk the same size as the internal disk, and it makes a bootable clone of the system disk, for what that's worth. The free version only does the mirroring, no incremental backups.

I don't know if you can do that or want to, but a fresh start seems called for.

Good luck.
If you go to the Geek Squad, make sure your disks are squeaky clean. They scan for "porn", in a secret alliance with the cops, and there have been instances where bare-ass baby photos have gotten people in big trouble.

I don't know if Apple Geniuses are any better or worse.

FWIW, the upgrades left all my files untouched. Though, it did try to move my home directory to iCloud. Upon upgrade, check iCloud Preferences and turn that off if you don't want your home in the cloud. I like it on the local disk.