Store any and all private data on a private server (rent something cheap for 5 bucks a month), restrict access to an SSH key that you store on two USB thumb drives (or remember a proper passphrase if you want to avoid the USB drives).
Once over the border, buy a new phone and rsync the data to it.
Clear the phone before travelling over the next border and either keep the phone for travelling or sell it and repeat the procedure for each travel. After passing borders like those of the US and China, where you (may) have to hand over your phone, you can no longer consider the phone secure. And no, wiping the drive or re-installing the OS is no guarantee.
On the other hand, phones can generally not be considered secure due to hardware backdoors, so why bother.
If you actually care about security, do what >>65931
said and don't bring a phone.