Brain wallet just means memorize the phrase but its more a last choice emergency situation where you have to get out and can get to safety fast enough that you won't forget your phrase

As for tangem I have only seen it in passing before. I can't figure out if the firmware is open or not. Its interesting for your use case cause Ican see how you might be able to hide it as a credit card. I would be worried about it not having a screen to verify addresses when you sign.