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.
I tried to do this a while ago with a GNOME system, setting GDM to automatically log me in, but I ended up always getting prompted for my password from gnome-keyring shortly after logging in which seemed to defeat the point. If you use GNOME, you might want to look at ArchWiki’s gnome-keyring page which describes a couple solutions to this problem (under the PAM section) which should be applicable on any systemd distro.
I’ve been using it since it felt usable enough in GNOME to me. Around 2015-ish, give or take a year. GNOME leading on Wayland support is a big part of why I switched to it from Xfce back then. Nowadays KDE and others have plenty good Wayland support too (better in some ways like allowing server-side decorations and global shortcuts) but I just haven’t felt like trying to properly experiment to see what I like.
I’ve always avoided Nvidia on my desktops. Stuck with either radeon or intel and never had any exceptionally big issues with them on Wayland. Though other things like hardware accelerated video decoding have had a history of being spotty on some drivers/GPUs.
Newer than C99? Both the Linux kernel and systemd build with gnu11. I’d call those pretty relevant.
C23 is still far too new (still a draft) for any major projects that care about compiler compatibility.