Rather than create a custom field type, use a select field and create a format value filter for the field to return the same thing that ACF returns for a taxonomy field. This would let you change the field type without altering any of the code where the current field is used and let you keep using it the same way. https://www.advancedcustomfields.com/resources/acfformat_value/ Probably a lot less coding than either of the other choices?
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