Review Large Attachments won't delete-no clue where they reside

Current issue is around clearing space on my 3rd Gen iPad, which lists 15gb of large attachements for Messages. I've tried to delete as many (painfully) individual files through that option in Storage (why is there no Delete All?) but it never takes. I've erased all pics and vids in the Photo app from Messages. Nothing. I've tried hooking up to my iMac/Photo but it says I'm "optimized" with iCloud.

I'm at a loss because I have an iPad, iPhone, iMac and Mac Book Air that access my messages, but do not have the storage on my iPad. The attachments don't seem connected to the cloud, but short of deleting entire conversations off the iPad I can't see any way to clean this up (will deleting conversations on the iPad delete them from my other devices?)

Its confusing to me because everything seems simultaneously connected but also disconnected, and Ive lost track of what action on what device will impact the rest.