Adding support for zstd is a fine move. None of this should be interpreted like that first comment of being due to "xz having a smell" about it. That is FUD, and this same kind of situation could happen to countless projects.

yeah, ok. So I just went back to test the S3 Sleep power state option, and switched it to the Windows'y one, which the BIOS says Linux supports and it does, but now the machine seems to still burn battery when it's suspended ... So, I have found the larger culprit.

In the bios of many thinkpads there are options about powering USB ports while the machine is suspended or powered off. I disable that, because I want battery life even when suspended. Also an option about some windows suspend or S3 suspend, I chose S3 suspend, but don't have clear evidence this choice makes a difference.

Big agree. I tried to make jellyfin acceptable for a while and life with audiobookshelf is just so good. https://github.com/advplyr/audiobookshelf

I've been on this hunt lately as well, but because I want to believe there could be a suitable option for a Linux tablet ...

Regardless, for just a new thinkpad the X1 carbon has remained a solid choice for all around work-flows (research, student, development, business, etc.)