backing up iCloud content when you only have an iPhone and an Android tablet (and forwarding email from iCloud?)

A friend of mine (no, really! :)) is in a conundrum with a choke-full iCloud account, and to my knowledge without easy access to a desktop computer.

I haven't seen a breakdown of which categories take up the most space but she'll need to make space urgently because her main email account is there too.

A few questions:

• what's the easiest and most efficient way to download all content from the Photos and Drive (and whatever else is *big* in her account) to an external (thumb) drive? Again, she has an iPhone (a 13 Mini IIRC) and an Android tablet.
• in the meantime, is it possible and useful to set up the iCloud email account to transfer all incoming mail to another account, preferably from the iPhone rather than through the web interface? With *useful*, I mean will email actually be forwarded to the other address is the inbox is full or will it still bounce?

EDIT: I have an A1440 adapter which does allow me to connect an external audio interface (so it *is* OTG and shows up as such in General/About) but it does not appear to support accessing the thumb drive that I just plugged in.

EDIT2: oh, I can via the Files app, but that only allows me to transfer the contents of the iCloud Drive...