Time Machine infinite stuck in preparing backup

Since TM has been a problem check this out...

Purging local backups

Please note that although this doesn't affect your remote backup from Time Machine, this will get rid of the redundancy (at least until the next Time Machine backup) that a local backup disk will provide. If you need such redundancy or are worried about the recovery of your data then you would be best served to let macOS determine when to purge these files.

Start Terminal from spotlight.

At the terminal type tmutil listlocalsnapshotdates.

Hit enter.

Here, you'll now see a list of all of the locally stored Time Machine backup snapshots stored on your disk.

Next you can remove the snapshots based on their date. I prefer to delete them one at at time. Once my "System" disk usage is at an acceptable level, I stop deleting but you can delete all of them if you want to reclaim all of the disk space.

Back at the terminal, type tmutil deletelocalsnapshots YYYY-MM-DD-HHMMSS , where will be one of the dates from your backup. This will be in the form of xxx-yy-zz-abcdef. Try to start with the oldest snapshot.

Hit enter.

Repeat for as many snapshot dates as required

http://www.thagomizer.com/blog/2018/03/27/cleaning-up-time-machine-local-snapshots.html

Hello and welcome to Apple Support Communities, SirRic.

If I understand your post correctly, you are not able to create a Time Machine backup because of limited available space. I’d like to help.

Follow the steps in this article:

How to free up storage space on your Mac

Let us know if that resolves this issue for you. 

Thanks for using the Apple Support Communities. Take care.

I turned off automatic backups, does this eliminate snapshots?

I believe it does but cannot test. :)

Go ahead & create a new topic if you wish, do you mean 15 GB free on the boot Drive?

BTW, I do not trust iCloud nor iCloud Drive.

iOS Device Backups

You may have spotted a MobileSync folder in the Application Support folder. This folder contains iOS device backups made in iTunes. (If you only back up your iOS devices to iCloud, then this folder won’t be very big.) You may have multiple backups of your iOS devices, which take up a lot of space.

In iTunes, choose iTunes > Preferences > Devices. There you’ll see a list of backups. If you see more than one for any device—some might be very old—right-click one of the backups and choose Delete. This could save a couple of GB.

OmniDiskSweeper shows you the files on your drive, largest to smallest, and lets you quickly Trash or open them.

https://www.omnigroup.com/more/

Forget the "Storage" report as far as categories, it's quite unreliable, but here's my Daisy Disk report...