I can’t work out how to delete tags or the old groups from MailChimp on their site. Last time I looked The docs say if you do delete a group you delete the people with it. And groups that have been used are not deleteable anyway.
Careful deleting people on MailChimp as you can’t programmatically add them again for data privacy reasons (according to the docs). When someone unsubscribes or you do it on their site a webhook on your church site is fired which changes the setting for that person to not wanting email (which is the behaviour a user would expect when they unsubscribe)
The MailChimp API is a bit of a mare. What we have is the best I can do with it thus far.
I suspect having lots of tags and emailing very small subsections of a list is not what they were envisaging!