I’m not sure if ACF has a field like this ( not that I could see in the basic version anyway ) but this type of thing would work best as a Non Hierarchical Taxonomy with a Select2 type field. Think post tags: it’s searchable, you can assign multiple, and it’s listed out in a formatted easily manageable input for the user to digest. You can control whether the user will be able to create new terms or only select from existing terms. You can populate with AJAX and give them a populated list of options. It seems to make sense for the usecase described.
Terms is pretty efficient when querying because of the relational tables already setup. A custom table and query may be more efficient but would also be more work to setup.
