You can make it technologically impossible, but they can also come and arrest you just for using such technology. So its not really a technical problem, its a social/political one.

I don't think there's a way with a phone that people would actually be willing to use. At some point it has to be decrypted to be displayed to the user and there's always the chance there's a flaw somewhere in the stack from hardware to OS to app etc that will have a gap to exfiltrate the data.

There are no technical solutions to human problems. This has been explained over and over again, most famously in Randall Munro's XKCD comic where the secret police resort to hitting someone with a $5 wrench until they give up the password.

If you're in a repressive state and you're worried about your data being exfiltrated the best security practice of all is not to create records of illegal activity. If you have to store such material, don't keep it on a communications device, put it on an external storage device, hide it somewhere outside your home, and don't tell anyone about it.

Avoiding centralised services is generally a good start. You could also do something like encrypt any messages through PGP even if the service you're using is already "e2e encrypted" like iMessage or signal

The problem is they'll legislate for the providers to insert back doors, negating cryptographic hardness.

https://xkcd.com/538/ User data can only be as safe as the user.