For the TPM/Evil Maid part of your question, we intend on porting PureBoot over to the Librem 5 so you will get similar tamper detection of the boot firmware and /boot files.
With respect to travel, the best advice I can give for protecting against exposing sensitive data either from lawful seizure or from theft when traveling is to travel without the sensitive data to begin with, if you can, such that you can 100% comply with any customs or border agents in whatever country you are visiting.
A lot of geeks have fun putting on their “international spy/smuggler” hat and come up with a lot of elaborate measures whether it’s hidden partitions, steganography, or panic codes that wipe their storage. The problem is that they aren’t international spies, but customs agents in every country are professional human lie detectors. People try to lie to customs agents every day so they can traffic drugs and other contraband across borders. Your average geek is kidding themselves if they think they could somehow smuggle data hidden on a phone across a border without showing the kind of non-verbal cues these agents are trained to pick up on. Not complying with a lawful search risks your being detained and most likely means the electronics themselves will be confiscated. The best plan is one where you can fully comply and lose nothing.
Beyond the movie spy threats, the more common threat is that you lose your phone while traveling, either accidentally or because of theft. You are at a much higher risk of any of these things happening when traveling so ideally your phone would only contain the information/applications/etc. that you need while traveling and wouldn’t contain information you don’t need.
For what it’s worth, I’m personally working on an approach to address concerns about losing sensitive data while traveling. While I’m not quite ready to announce our approach, I figured it was worth letting you know that first, it’s something that we are thinking about and working on, and second, my general philosophy about the best ways (and worst ways) to tackle these problems.
Keep an eye out on our blog. I should be ready to announce something in the coming months.