System taking up 86GB - run CleanMyMac, no idea what else to remove?

First of all, uninstall CleanMyMac as per the developer's instructions. No so-called “cleaning” apps are needed or recommended for Mac OS. They can conflict with Mac's own built-in security. CleanMyMac has been known to delete important system files.

The Storage tab frequently does not report correctly. Also check by choosing your Mac HD and "Get Info" (command-i). It may be different. Try rebuilding Spotlight index.

How to rebuild the Spotlight index on your Mac - Apple Support

Some of the space may also be from Time Machine Snapshots.

About Time Machine local snapshots - Apple Support

After you've freed up some space download Omni DiskSweeper:

https://www.omnigroup.com/more

DiskSweeper can give you a more accurate read of disk space than Finder or Storage Tab of About this Mac. Itcan also show you the precise size and location of all your files. It will inventory your disk starting from the files that take up the most space. If you want you can even delete files from OmniDisk Sweeper. But be careful some of the items may be important to your system.

Also, just running ODS should fix the storage number.

Check Mail menu > Window > Connection Doctor. If you have check marked “Log connection activity” turn it off.

For information about the Other category in the Storage display, please see Apple Support Topic

https://support.apple.com/en-us/HT202867

Also, empty the Trash if you haven't already done so. If you use iPhoto, empty its internal Trash first:

iPhoto ▹ Empty Trash

In Photos: File ▹ Show Recently Deleted ▹ Delete All

There are no cleaners you should subject a Mac to. There isn't one that doesn't delete things it shouldn't.

Uninstall apps in only these ways.

1. If it's from the App Store, simply but the main application in the trash.

2. For other third party software.

a) Use the uninstaller provided by the vendor.

b) If there is no uninstaller, check the vendor's site for manual removal instructions.

c) If a or b is not available, contact the vendor and ask them how to remove their software correctly.

I notice you have Norton Security installed. That is also a 100% piece of junk you should remove. There is no AV software you should install on a Mac. They are all worthless. Unless a person believes slowing your Mac down is a good thing.

Hello,

This is a known problem. The "System" portion of your reported storage can be anything from 90 GB to 2.5 TB. It will virtually never be close to the correct value of 35 GB.

The only thing you can do is ignore it. There are some tricks to get it back to the correct value, but it will be back up to 100 GB tomorrow.

But if you haven't made a backup since 2015 - that is a MAJOR problem.

CleanMyMac is complete garbage. Uninstall it completely.

Until you do, any troubleshooting is completely pointless.

Remove CleanMyMac. It will slow your Mac to a crawl.

How To Uninstall CleanMyMac 2? Complete Removal of Mac Apps

Better to download and install third party so called Mac cleaning utility. They can actually do more harm than good.

How to free up storage space on your Mac - Apple Support