Btrfs doesn’t have encryption, so you need to do it with luks to an mdadm raid, and build btrfs on top of that. Luks on mdadm raid is known to be slow, and in general not a great idea.

Why involve mdadm? You can use one btrfs filesystem on a pair of luks volumes with btrfs’s “raid1” (or dup) profile. Both volumes can decrypt with the same key.

A standard called SystemReady exists. For the systems that actually follow its standards, you can have a single ARM OS installation image that you copy to a USB drive and can then boot through UEFI and run with no problems on an Ampere server, an NXP device, an Nvidia Jetson system, and more.

Unfortunately it’s a pretty new standard, only since 2020, and Qualcomm in particular is a major holdout who hasn’t been using it.

Just like x86, you still need the OS to have drivers for the particular device you’re installing on, but this standard at least lets you have a unified image, and many ARM vendors have been getting better about upstreaming open-source drivers in the Linux kernel.