I am not sure if I have the issue correctly undertand, but it seemed you want to create a microflow, a retreive action for the related entity and add the filter for your data. I hope this helps.
When you use a database retrieve then you can use xpath to filter for elements. If you use a retrieve by association then you need to use list operations to filter.
In Microfolow you have a Retrieve Objects activity that can be used. YOu can further add filters to it in the form of xpath constraints to filter the result data .
You can also go through the below link to get some idea :