2tb should be all you ever need, even for a gigantic comic collection. Move other,non calibre stuff to your cloud and keep calibre local. You can also backup calibre into cloud but don't run it in there.
As for your original trickery request, windows has symbolic links which can fool a program into seeing networked storage as local, but I don t know the equivalent for Ubuntu. And it's safer not to do that for any calibre stuff
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