How do I archive messages and free up iCloud space without losing data?

I’m at my limit on iCloud because 90% of my space is used by Messages. My goal is to clear that space, but keep a non-iCloud archive of those messages and attachments. (I am not looking to purchase more iCloud storage.)

I cannot seem to get a straight answer if this is possible.

I understand that I can save and delete individual messages (https://support.apple.com/en-us/108922), but I have hundreds of threads.

Likewise, I understand that you can manually delete attachments (on an iPhone or iPad only.) But again, you do this literally one-by-one – no select all option.

Further, I am not clear what happens when I turn off the option to sync messages to iCloud. Does that iCloud space immediately become freed up? If not, when does it free up? Do my individual devices keep the history of all those conversations? 

If anyone has any suggestions or information, I would appreciate it.

Separately, for anyone at Apple, I get that the goal is to get people to purchase more iCloud storage, but this seems purposely obtuse and user hostile.

[Re-Titled by Moderator]