bulk transfer of ALL iMessage attachments (photos and videos) from iPad

There is no way at all to bulk select all attachments in iMessage to copy or save elsewhere. The best that you can do is to select each one individually and then save them after they have all been selected. You can only do this for one contact or one conversation at a time anyway, so it will take some time.

I know of no cloud solutions for bulk saving texts and the attachments. There are third party apps for a computer that will let you do that, but I’ve never heard of being able to backup them up to any cloud service.