If you're in a position that your chain of custody is broken, just wipe, revert the device to stock signed binaries, confirm by locking the boot loader again, then unlock and re-flash your custom binaries.
Or am I missing something here?
FBE is only secure when the system partitions have dm-verity enforced and verity is only enforced when the bootloader is locked.
FDE is a bit more resilient with an unlocked bootloader since there is less data left unencrypted.
However in either scenario without a locked bootloader and verity it is trivial for an attacker to insert malicious code that can then run once your device is unlocked and send off your files.
(I believe this is an example of an evil maid attack, although that name for it is somewhat new to me.)
A locked bootloader would prevent such code from being installed, or at least refuse to run it.
https://www.reddit.com/r/LineageOS/comments/ev9c4v/is_androi...
I really have enjoyed the LineageOS and it's been great to have a phone that isn't as dumb as a flip phone but doesn't have all the bloatware and spying of other devices. The security around the bootloader is my biggest concern.