Well, since no one can answer my question let me inform you what I found out. First of all, our Portal instance is a 2 tiered system that sits on our intranet (behind firewall). We are using ArcMap 10.7.1 to publish services to our Portal.
Scenario 1
Relate a stand-alone table to our parcel layer (ArcMap) and published to Portal. Data was copied to our datastore (PostgreSQL). The results were very poor. After enabling popups it was hit or miss when trying to access the related content. Very frustrating.
Scenario 2
Add the related table and feature class to file geodatabase and move the file geodatabase to ArcGIS Server and setup datastore to folder. Again, poor performance and the relate is not persistant.
Scenario 3
Add the related table and feature class to file geodatabase, create a relationship class. Data was copied to datastore. This worked the best. Relate data was more stable and configurable within popups.
So, to sum things up (since no one else could help). I would highly recommend using a relationship class when using a related table. Also, publishing to our datastore (PostgreSQL) allowed me to customize some of my popups using Arcade expressions.