On the theory that the upload 500 error is because the media directories aren't synced with the bucket, I'm doing a dry run of the sync and wow... the sheer number of cached avatars & headers from remote servers is... concerning? It was nothing but avatar caches for a few minutes now, and it's moved on to headers just now. The lack of any user-level mechanism for cleaning these files up has been pointed out as a problem, I believe.
Accounts culled, though it doesn't seem to have done a huge amount disk space-wise. Running media file synchronization now.
Wow cache is far and away the biggest directory of media files so far, and accounts/avatars alone takes up 4.9 GB. And that's with a blocklist a mile long including the biggest instances at mastodon.social and .online. No wonder media cache cleanup did so little, since it evidently doesn't do anything about cached remote avatars. Very glad to be getting all this off the server's limited disk space--now if only I could get the server to serve these images! :P
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