Yeah file management in general is still incredibly rough around the edges. I would still like to be able to simply grab files from my phone via a connected linux computer, but apple will never let me have that one.

>no way to adjust what data gets cached or when it gets purged aside from rebooting

Unfortunately the only current ways to pull this off is to either artificially move the current time years forward so it hits a time gate and dumps the cached files, or use enough space on your iPhone to force it to dump the useless cached files then delete whatever you used to take up space temporarily.