There should be a way to use the SSAS Tabular Connector without AD Sync, e.g. by using CustomData on the connection string.
See my prior comments. I wonder if Teo would be willing to alter this idea to not just be about working without Active Directory Sync, but to also address the scenario where your AAD UPN does not match your on-prem UPN thereby causing the connector to be unsuccessful when connecting to the on-prem SSAS server instance.
