Encrypt cache files on Android

Hi there !
I’m using Nextcloud on my NAS and Android phone and I find it really usefull, but I’m using it so I can have nothing on my phone in case I lost it or someone steal it from me. So the fact that the Nextcloud app stores the thumbnails in clear is pretty problematic for me. Is there a way to store them encrypted or someway so someone going through my files won’t be able to see them ?

P.S. Sorry if I’m not in the right section of the forum.

I just found this, can’t you encrypt the phone via the operating system:

I know about his, but I don’t want to try it has it is irreversible. And even if I encrypt my phone, I’m pretty sure we can still see the thumbnais in clear with a file explorer.
As it isn’t that hard to unlock a phone with fingerprint nowadays (they are all over the phone :confused:), I would really prefere the thumbnails to be encrypted.