Problem:
We would like to have an option to control the related entities that show in the Related section of Advanced Find for a better user experience, otherwise that list gets very long and the user cannot really find what they are looking for.

Suggestion:
Either the setting on the lookup attribute could control the filter both on the main filter section as well as the Related filter section, or there could be an option on the relationship itself to control the Searchable property of the Related section in Advanced Find.