It's only taken me some months to get around to trying your suggestion and I can confirm it works! Anyone that encounters permission issues needs to do this to ensure any documents and folders synced down from OneDrive have the right permissions. So to simplify it for all the other noobs (like me) out there just seeking to sort out permissions, here is what I did on my Windows machine.
Go to https://raw.githubusercontent.com/abraunegg/onedrive/master/config and copy the content into Notepad.
Remove the # to uncomment the following parameters and adjust the permissions to sync_dir_permissions = "777" and sync_file_permissions = "777".
Save the file to the config folder you created in the OneDrive docker container and simply call is 'config'. Note, no extension.
Restart the OneDrive docker container and do a test run by creating a folder in OneDrive, adding a file to it and wait for it to sync to your server. Test that you can open the folder and the file on your Unraid server. At this point, you should be all set!
Thanks to @cholzer for pointing me in the right direction. This has been bugging me for a long time but I usually sync up from my Unraid server to OneDrive rather than the other way, so it didn't bug me enough to do anything about it until now.