Well, in the EU you must declare everything over 10.000€.
I would say that in general it is not legal for them to go though your apps but I have seen border control programs where it does seem mandatory, or at least they can make it more difficult for you to enter the country or hold you if you don't cooperate.
I was thinking hypothetically.
I would use an electrum wallet
I would use a 13 word seed.
it has that option.
I would send multiple emails to multiple email accounts that I control
in each email i would have two or three of the seed words.
alternatively i would put the list of 2000+ words for seeds on this thread.
I would number the seed words 1 to 2000 plus
I would send emails to my emails with the numbers instead of the actual seed words.
either method above is okay for a temporary move.
certainly not perfectly safe.
you could fuck up when you create the emails and lose the 13 word seed.
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This is quite a long and risky process Phil especially if there's quite some fancy amounts on the address.
I think in a situation like the the old and regular paper wallet is nice enough. You can write those seed on a piece of paper, open up your pc and keep it in there.
Alternatively get a second device, format it don't connect it to the internet, boot in your wallet.
When you get past customs you can create a new wallet and moved your coins to ensure their safety.