10.13.1 won't update

I have tried everything I can find with troubleshooting on my early 2015 macbook pro. I downloaded, installed, and restarted my macbook pro to update from 10.13.1 to 10.13.6. It went through ALL the motions but NOTHING'S CHANGED. Under "About This Mac" it still shows 10.13.1 and I've tried:

-Freeing up more than enough GB of storage space on my SSD. I had over 60 GB available when I installed the update.

-Repeating the download & install process

-Shutting down & restarting

-Creating separate Admin user to install update

-Rebooting computer using Command+R

-Rebooting Computer using Safe Mode (it reboots in Safe Mode, but the display then glitches so badly that my internet window goes white and I can't even get to the website to click on the download).

-Downloading a Combo update

-Rebooting using Option + Command + P + R (or was it Shift + Command + P + R? Something like that)

-Recovering and using my backup from external hard drive

-Opening terminal and using command "softwareupdate -l --include-config-data" which just shows me updates available for install, but I have no clue beyond the vague user tip of "just use -i to download one by one" because I don't speak/write code and just using "-i" doesn't work for me in Terminal.

NOTHING IS WORKING. I can't just go out and get a new macbook pro because the program I need that required the macOS software update in the first place is Pro Tools First, and Pro Tools First does not support Big Sur.

I'm overwhelmed and beyond frustrated with this issue. Please help!