Someone might take pictures of benches and another one will add the info if there’s a backrest from home.
What about https://mapcomplete.org/benches ? Granted, it does not upload to Wikimedia commons - but then again, Wikimedia commons isn't the right place for thousands of (mediocre) pictures of mor or less identical benches. A picture there should be "qualitative" and either "educational" or "relevant".
What do you mean with "it's not accessible to the broad public"? The pictures get linked properly with an 'image'-tag, so it shows up in e.g. OsmAnd.