Remove integer validation that does not work when slugs are used. Using slugs is OK for wp_set_object_terms() Docs. Also mixing with array seems no problem. We can rely on the API to take it.

I simple added the filter at the end anyway so the filter can be used at all time even if you are using a select-menu field.
I don't know how that would change the interface to add hooks or fields but that does not seem to work at all.

With that code i managed to get my custom dynamic dropdown using slugs inserted into the post.