Debian, like any other legacy distro, mush became declarative, because the '80s model of manual deploy and the absurd pain of D/I and Preseed must end.

bootcrew have bootc Containerfiles for Debian, Ubuntu, Arch, and openSUSE:

https://github.com/bootcrew/mono

In the end, Nix is just a thin veneer on this stuff.

Given how many quick & dirty sed patching or exec commands I've seen in the few nix package/modules I've read, I would not exactly bet my life on it being completely idempotent & reproducible.