Tried nixos-raspberrypi out.
Apparently a significant benefit is you get a chose of bootloaders. And uboot - required by nixos-hardware - supposedly has an issue on rpi5 in that USB devices such as keyboards don’t work during boot (this is what LLM told me; I haven’t verified this).
But I noticed the kernel version is old, so I filled a bug report. kernel version old and insecure · Issue #191 · nvmd/nixos-raspberrypi · GitHub
If I need to manually update the kernel, I lose the benefit of the caching, and it probably isn’t worth while for me.