Oh, my, yes.
I'm afraid I have only 2 suggestions. One is in the case of attached images, the other uses the ArcOnline Map's customized PopUps. From my experience thus far, any other solutions require Developer knowledge/experience -not my forte, sad to say.
For the dynamic display of particular attached images, see my previous post.
For the use of customized PopUps, you can display the customized/formatted PopUps in the Feature Info widget. So, in the ArcOnline Map being used to source the ExB app, configure your PopUps using Arcade Expressions. The following arcade script will return a list of related content:
You might not need the count variable, but I used it to check if a particular item had any related content. It's also essentially the same as what ExB's default Count for Related Data does.
Here's an excerpt from p.6 of my Documentation of Project Development... from around last August(?):
...Ah, memories. This was from when I began to think I knew what I was doing...
This may or may not be helpful to you, or go some way to answer your question re: displaying related data dynamically. ...It's all I've found so far.
If the data is truly related you can have a look at the FeatureSetByRelationshipName function that allows a more direct access to related data. https://developers.arcgis.com/arcade/function-reference/data_functions/#featuresetbyrelationshipname