Thank you for your post and I'm glad that you were able to resolve your issue! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others", I'll repost your solution in case you'd like to accept the answer.
Error Message:
DeleteCloudOnlyObjectNotallowed
Issue:
After receiving the above error message, you utilized the synchronization service manager to gain a better understanding of the issue. Even after deleting the user(s) within Azure AD and allowing time for a re-sync, you were still receiving one error with no details.
Solution:
To resolve the issue, you queried for the user using the Source Anchor value and found the user.
If I missed anything please let me know and I'd be happy to add it to my answer, or feel free to comment below with any additional information.
Additional Link - DeletingCloudOnlyObjectNotAllowed - Azure AD
I hope this helps!
If you have any other questions, please let me know. Thank you again for your time and patience throughout this issue.