[angry_octet]

More obscure phones might not have support, but that doesn't mean they can't be opened with more effort.

When off, you're relying on the strength of the FDE passphrase and whatever key strengthening they implemented, and that the OS didn't leave some key fragments somewhere (accidentally on flash, which would be very bad, or remanent in memory if it has only been off for a short period).

Using a long alphanumeric (>12 random, >20 passphrase), not installing random apps, keeping it patched and keeping it powered down is probably the best you can do. I wouldn't use the baseband comms if I could avoid it, just a huge 4G attack surface.

[Forbo]

Android has a 16 character limit for your password. Or at least it did as of Android 10, not sure if that's changed.

[marcan_42]

If you root your phone, you can set your FDE passphrase to whatever you want while keeping a usable shorter unlock code. My phone's FDE passphrase is 26 characters long.