Data Storage

I have an old (Generation 3) iPad, with iOS 9.3.6 (latest for this iPad). I just use it as a book reader now, and for one game. It has gotten slow, so I am clearing out all useless data. I don't need it for emails, messenger hasn't worked right for years, etc. I've deleted all emails and removed the email accounts; all messages, and signed out; podcasts are not synced; all photos are deleted; calendars cleared and stopped syncing, etc.

And yet, in my Manage Storage settings:

Messages 80.0MB

Calendars & Reminders 7.2MB

Podcasts 3MB

How do I clear this junk? Why doesn't "Manage Storage" actually let you MANAGE STORAGE?