Posting here as a last-ditch attempt to find a creative solution to my issue.
I created a query layer that joins point geometry to a related table. (The relationship class is 1:M, but there is only one related record of this type associated with the point.)
Symbology and labeling in the map depend on the data in the related table. I published this query layer as a view-only feature service to Portal version 11.5. No problem.
Now, I need it to participate in our distributed collaboration to ArcGIS Online (as a copy, not a reference). When I Enable sync and try to publish from Pro, I get the error message "Dataset ... is not configured for or cannot be used with the Sync capability: Publishing a non-versioned dataset without archiving enabled by reference." This is logical: the system has no way to check for changes in the data without either versioning or archiving. Both the point feature class and the table have archiving enabled, but that doesn't come through in a query layer.
So then I thought I'd try to use Arcade to get the information from the related table, but FeatureSet functions don't work in labeling or symbology profiles. Again, this is logical: first, the data would have to be queried, then it would have to be rendered. Translation: s-l-o-w performance.
Can anyone think of another workaround that I am missing? The labeling is what really gets me - I'd really like to have a label so that the user doesn't have to click on the point to find out what is there.