I was able to figure this out, turned out to be a simple answer of course! The taxonomy field returns value in an array (for example $_POST[‘acf’][‘field_5b7cf5019f615’]), I was able to use this to retrieve the ID of the term selected.
The search form I outlined above worked out well. I was able to use the pre_save_posts field to store fields values from the form in the session and use them to search my publications archive. I was also able to use the load value filter on the taxonomy field to keep the last term searched selected.
I continue to be impressed with how thorough this plugin is, lots of tools available when you need something somewhat custom like this.
