Makes sense, I'd probably prioritize pruning old backups first too. bup looks quite cool regardless, I'll be keeping an eye on it.
I like & use Duplicity, but it's a little finicky to set up right and some of the more obscure features are counterintuitive or even broken.
some ppl have reported using an encrypted storage backend like ecryptfs to store their bup repositories in. that option shouldn't be too hard to put together.