Arf, that's exactly why I 100% prefer on-prem SharePoint!
In your situation, in all cases, you'll have to setup and maintain another server. Your options are:
- Create a Windows Service hosted on a third-part server. You can write in C# and leverage the client-side .NET API of SharePoint.
- Create a Console application, still hosted on a third-part server. You can also use the .NET client API of SharePoint. Schedule it with the Windows Task Scheduler.
In both cases, you'll have to configure credentials used by the tool to logon SharePoint. All actions will be done as this identity (no
SystemUpdate, no elevation of privileges, etc.)