Field updates - do not overwrite unless by API

I've been searching the community to see if there's a way to allow fields to be updated by API (regardless of if it's populated or empty), but restrict field updates when importing a list. Ideally, when uploading a list, I'd like to be able to select fields to update if blank, but not overwrite if they already have a value. Anyone know of a way to do this in the settings?

At first glance, this seemed like it would work, but I still want our API to update (based on our database) regardless. Any ideas?