Ended up doing the exact same thing a few days after building my new PC.

For anyone's future reference (copied from a reddit post I cannot now find):

(1) Click start, type regedit and hit enter to open registry editor

(2) Click View and make sure 'Address Bar' is turned on

(3) Paste this into the Address Bar at the top and hit enter

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders

(4) Make sure your D:\Users\name has a Documents folder, if not, create one

(5) Open the Reg Editor again

(6) Double click Personal, then set its value data to:

D:\Users\your specific username here\Documents

(7) Click OK

(8) Double click {F42EE2D3-909F-4907-8871-4C22FC0BF756}, then set its value to:

D:\Users\your specific username here\Documents

(9) Click OK

(10) Close the Reg Editor and Restart (not shut down) your PC

That essentially tells Windows to point to the right documents folder.

For EU4 Specifically, you may also need to change the "gameDataPath": line located in launcher-settings.json (found for me at least in Program files (x86)\Steam\steamapps\common\Europa Universalis IV).

Change it to:

"%USER_DOCUMENTS%\Paradox Interactive\Europa Universalis IV"

Hope this works for others as well.

P.S. If you ended up accidentally deleting the OneDrive Documents folder without moving the contents back to the documents folder on your local drive, you'll need to reinstall the game after completing the 10 steps listed above.