The out of date problem you mention is something Nix contemplates. As of a couple hours ago, if you try to install Etcher (Balena Etcher, that is), you will get a message saying there’s an out of date dependency: Electron.
You can still try to install End of Life packages, but you have to type an instruction the ‘compiler’ tells you to type out. In other words, you have to be deliberate about installing out of date dependencies.
The other problem I understood it as “if I declaratively do this and then imperatively do that, how do I keep track of it all?”. If that is indeed what you’re asking, you’re right. That’s a problem. That’s why I try to keep everything in the config.
I’ve been using Nix now for a couple of months and the configuration file has been a great friend. Sometimes I don’t know how to configure something and I get lost and worried, but the community has helped me to fix my problems- .