I was able to get the form to show by altering form-topic.php, but it doesn’t register the tags after submitting it. Assuming it’s because ‘assign_topic_tags’ is not set anywhere for anonymous. Works fine for logged in users.
Thank you for your help. Unfortunately, that doesn’t work. The topic gets submitted but without topic tags. I think user caps get checked somewhere else, as it works with any role including custom ones.