From what I see from several answers, I may not have explained myself well. Going on vacation and carrying less than the legal minimum is not a problem. I was thinking more of moving to another country, and taking the holdings you normally manage with your Hardware Wallet on your laptop or PC.
I mean, carrying 0.01 BTC on your mobile is not a problem but if we are talking about 0.5 BTC that you manage with the HW....
Although I see that this is pretty hypothetical, according to the answers.
I had also thought of taking the seeds written in a book or notebook on several pages, but with invisible ink, so that no way that it will occur to customs agents to search through a book of 300 pages.
If I were you, this is what I would do. If I were changing living, I would probably risk moving with a Hardware wallet but since this can be an issue, I would go this way - Your t-shirt has a label tag inside. Create a wallet with 12 seed phrases, transfer coins to this wallet, then write down 12 seed phrases on your t-shirt's label tags (mine has multiple) and when you move in another country, buy a brand new hardware wallet, restore the wallet and destroy the t-shirt.
Be frank, isn't it very creative? And safe?
Well, in the EU you must declare everything over 10.000€.
You can also not declare. Who is going to know that you hold Bitcoins that are worth more than 10K euros? No one will know. That law also applies exclusively to cash as far as I know.