Okay, after doing further research, I think I can provide further information and an actual solution to my problem.
Initially I thought occ:decrypt-all it does nothing, but it actually does what I wanted all along but misses a crucial step: It doesn’t update the information in the database. The server still thinks the file is encrypted. You need to manually tell the server to unmark the files…
UPDATE oc_filecache SET encrypted = 0 WHERE encrypted = 1;
Otherwise, you’re going to experience the error I experienced
Error webdav Sabre\DAV\Exception: Unable to get header size for <redacted_file_name_that_is_totally_not_secret_and_should_ideally_be_encrypted_too...>, file doesn't start with encryption header at apps/dav/lib/Connector/Sabre/File.php line 600