Very well said. just wipe your device and then if they say "why is this phone wiped'? There's so many plausible excuses. You just bought it. It's a backup device. You haven't had time to set it up. Definitely wiping it after they get their hands on it. Not a good idea.

How do you set up and access that remote back up?

There's a variety of ways to do that. The clean phone would generally make ssh key auth hard, though if an ssh key is the only thing on your phone you're better off than all of your data being there. My approach is to keep my data close to me, so an ssh/sftp server on site is my preferred approach, secured as much as you want. Then just use something like AndFTP and maybe Termius to get your data. Making it smooth is the part that can be harder if you want to just grab a whole phone backup and make it like you never wiped. I'll admit I could stand to work out my own protocol for this -- I don't do much cross border travel. You can make this more complicated if you want, especially if dealing with dynamic dns. But sftp in general is the easiest way I've found for handling secure file access, at least with stock android. Sshfs may be a viable option too to just access rather than download the content but last I knew that did require root access to set up.

You can also just use passphrase login. Less ideal, but can be easier. You determine your own level of paranoia. Heck you can even run an OS in a browser these days. Make your phone a dumb amnesic terminal and never worry who accesses it.

I may have assumed too much knowledge too. The actual software you'd use to host the SSH server comes standard on Linux systems, but can be installed on windows and OS X (if you must). It's called OpenSSH. The Android apps for interfacing with it would be AndFTP (at minimum) and Termius (optional but handy as this will also let you have command line access and also can set up SSH tunnels which can be thought of as a poor man's VPN, and are very handy for pivoting through an SSH server perhaps to a file server that itself isn't even on the routed Internet).

If you're contending with a dynamic IP address noip.com will give you a free redirect and there are scripts you can automatically run to update in case your address changes. Using tor is also a good way to address dynamic ip's but using onion addresses in android apps is not always a smooth experience.

As I alluded to you can wrap this in all layers of paranoia but the general ssh server configured with sftp and an address you can reach it at is sufficient for just not letting border patrol see your data. Everything else is just further hardening if you really are more of interest.

The app solution may already exist for rooted phones anyway (and be impossible on unrooted phones). OrangeFox Recovery looks like it will do this, to an extent that a restored recovery file brings you back to a point where logged in account sessions are still live. Not quite the same as Norton Ghost for phones but does look nearly as good.

Those that take privacy really serious don't have cell phones.

Yea...there's a spectrum, as with most things. Arguably having a dummy cell phone is less likely to draw as much scrutiny than showing up without one. 'I don't have a cell phone' said nobody under the age of 70 honestly. But anything you have at the border is a lot more out in the open than anywhere else you have it. And data goes over borders without needing to be subject to this state of affairs, if only you avoid physically bringing it with you.

I don't have a cell phone ... and I'm nowhere near 70. Got rid of it almost a year ago. Use one at home as a tablet, but I don't carry it with me.