Thank you for the reply @johtani. Regardless of the JSON type of the data i was inserting, I wanted to ensure that anything non-object value that wasn't explicitly suffixed with a type (as per my index template), would automatically get typed as a string.
It seems like I've been able to solve my problem by doing a few slight modifications to the index templates. I went from this 1st version template to this 2nd version template.
I suspect the "nested_items_template" section in the 1st version was allowing ES to automatically convert "timestamp": 1234567890 to a long as it's JSON type was a number (as you pointed out) and I was also using the {dynamic_type} value for type. I updated that section in the 2nd version template to only automatically detect object types and statically set them to "object". It seems like new values that don't match my indicated types (with _TYPE) or the object type, now match on last "string_fields" section, thus properly setting it as a string. I'm not an expert when it comes to index templates although I would appreciate it if you can in fact confirm this is what's occurring?
Thanks again.