I really don't see any problem with paper wallets, hot/cold wallet on a phone, a seedphrase on a USD stick. I'm not sure where you're from, and I never seen this happening, so I really can't imagine them going through your phone apps. Is that even legal? Also, is this even something you have to do? I mean declaring your bitcoin holdings regardless of the amount?
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.