There may be some lingering bugs with the API’s vision field – i.e. I’m not 100% confident you can trust the result, see https://forum.inaturalist.org/t/cv-model-included-flag-bug/47991/12
However, you should be able to get a basically complete list from paginating through https://api.inaturalist.org/v1/observations/species_counts?expected_nearby=true&page=1 (also available as a v2 endpoint)