About this Mac > Storage shows more disk usage than hard drive: impacts Time Machine

About this Mac > Storage shows more disk usage than hard drive capacity. This screenshot below illustrates my problem. Despite my late 2016 MacBook Pro having 1 TB of SSD storage, macOS (Mojave, latest updates installed) believes that I have 2.26 TB allocated to "Documents".

In of itself, this is not a problem except it's impacting my Time Machine backups. Specifically, my 3TB external hard drive is full, despite my having only 3 remaining backups. Every time I back up, Time Machine spends several hours cleaning up old backups because it believes the disk is almost full. Given the above storage problem, I don't think it's a coincidence.

I've searched the internet for a solution, and came across this:

https://apple.stackexchange.com/questions/189083/mac-says-im-using-more-space-than-i-have

I tried all four solutions, except Safe Boot, which I couldn't get to work under Mojave. Apparently many users have complained about this on the internet. Here's a quick summary of what I've tried.

1. Emptied my Trash and tried rebooting in Safe Mode. Holding down SHIFT while rebooting after multiple attempts failed. In Recovery Mode, running nvram boot-args="-x" from the terminal and then rebooting didn't put me into Safe Mode.
2. Booted into Recovery Mode and ran First Aid on my MacBook Pro hard drive. Errors were detected and repaired, but my problem wasn't resolved.
3. I tried reindexing Spotlight by adding my hard drive to the exemptions and then removing it again.
4. Running sudo mdutil -E / from the Terminal

Do any of you have any ideas on how to fix this?