It's a nice write up. but as much as I love whole-drive filesystems, in this case I would have used a partition table and used a fixed partition for the swap space. Not only is it (slightly) more efficient, it's also simpler than using a btrfs subvolume and remembering to +C the swapfile.

I think the general approach is very good and could probably be used for the VisionFive 2 RISC-V SBC as well.