Appreciate the quick response. The workaround didn't work. I started a new plugin folder "C:\Users\evanr\Documents\SimCity 4\plugins" and I can now download and update using the sc4pac.
For Steam users encountering the same problem, you'll also have to modify the game launch to execute from your new plugin folder. See below.
The steam's ".exe" file doesn't allow you to edit the target location in the .exe's properties. To get the game to reference your local plugin's folder:
1. Go to your Steam dashboard -> Library
2. Right click on SC4, choose properties.
3. In General, you'll see a box at the bottom that says "advanced users may choose to enter modifications to their launch options"
4. Type in the following: -UserDir:"(your filepath)\". It requires the quotation marks around your filepath, inclusive of the final slash. In my case, that is -UserDir:"C:\Users\evanr\Documents\SimCity 4\"
NOTE: I moved everything that was in my original SC4 one drive folder into the new local document folder, which included folders for plugins, regions, http cache, and a random "SimCity4.cfg" file.
I only know enough kung fu to get into a barfight, so I defer to the experts on here if there is a better way for steam users to redirect the game to plugin folders.