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 you live in EU and move within EU, then you won't have problem with customs because the only thing they care about are guns, drugs come the next thing.
By the way, I like the idea of writing down seed phrases on paper with invisible ink but I wouldn't carry invisible ink with me, I'd only carry a notebook. You can also write things down on supermarket bill and carry that with you in the bag or in the pocket. T-shirt method also doesn't sound bad. I think you have to try many safe methods. If I were you, I would do both, invisible ink and t-shirt method but on top of that, I would also try to memorize 12 words seed phrase. Now you have 3 backups, that would give me a piece of mind in this case.
Memorizing isn't hard, especially of 12 words seed phrase but don't do it the day before you fly, do it weeks before your flight. Exercise every day, repeat those 12 words every day for multiple times, for hours and it will be sealed in your mind. Read this 12 seed phrase like it's a song and it will be like a muscle-memory.