How about you set a storage time depending on the forum it is posted in?
So post something in AR or CN and 28 days later it expires and is removed.
Maybe the Lounge gets 28 days as well.
The other discussion boards get 6 months because those conversations are more important. Anything older and you are in necro land anyway.
And the FE boards get kept forever as it is an archive of flat earth stuff that can be used for reference at any point in the future.
Just make those the T&Cs. I think most people would accept we don't need to keep AR stuff forever, its just a convenience of a service. And if you want to make a lasting post down there, well use a third party as we don't think as a site that AR images need to be maintained forever at a storage cost.