Hi Colin,
As per your description, I tested in my side when I sync SharePoint online document library to local OneDrive for business sync folder and then when I try to remove “-Document” in local sync file explorer is get same error message as you encounter . See image below for your reference.
We would like to explain you we can change or rename a “-Document” name to any name via sharepoint online site setting but we cannot remove part “-Document” as like you mention only****Accounting", "Engineering", etc.
If you want to change a “-Document” name please go open SharePoint online site >click on Setting>Library settings>under general setting click on List name, description and navigation > there you can change any name instead of “-Document” and click to save.
To address your concern about remove“-Document “folders created from SharePoint online library the situation, we’d suggest you kindly feedback on this UserVoice. This is the best platform to let us hear from you and make our products and services better for you and others.
Your understanding and patience will be highly appreciated.
Best Regards
Waqas Muhammad