>I believe LUKS is capable of that
Booting into a 30 GB partition on a 128GB phone is going to be mega suspicious, even if the remaining data is random.
A partition based on a sparse disk image might avoid that.
You'd have what appears to be a 128GB image (or some large fraction of that), which in reality is largely holes (typically: repeated blocks of ASCII 00 bytes).
Of course, you'd need to avoid actually trying to fill that filesystem.
There are ways around it AFAIK.
There really isn't. You can try avoid the "30GB volume on 128GB phone" problem by putting the hidden volume inside the free space of another volume, but since the outer volume doesn't know about the hidden, you have to be very careful not to overwrite the inner volume contents. The safest way would just be to not use the outer volume, but that's also suspicious. Who carriers around a phone that doesn't have any activity for months/years? Not to mention trim/discard means that free space will get wiped by default, and disabling it also makes you stick out like a sore thumb.
You're better off traveling with a wiped phone, and restoring from backup after you've crossed.
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Right, it would have to be 64/64 for everyone.