I eventually did choose Svelte, but I’m dockerizing it so I can still use the thing a year from now, unlike virtually all of my other projects.
#!/usr/bin/env nix-shell
#! nix-shell -i fish --pure
#! nix-shell -I https://github.com/NixOS/nixpkgs/archive/4ecab3273592f27479a583fb6d975d4aba3486fe.tar.gz
#! nix-shell --packages fish gcc gnumake gmp git cacert
...build script goes here...
--pure: Environment is as pure as it can reasonably get.
--packages: Packages from nixpkgs to be made available in the environment.
-I <github_commit_hash>: pinned to a specific git commit of nixpkgs. This script will give you the exact same environment with the exact same packages with exact same dependencies forever.
You can drop into a shell with the exact same configuration by using the cli command nix-shell with the same options.
Admittedly this is not as declarative as using flakes, since it's a script, but hey, I'm lazy, still didn't sit down to learn them once and for all.
Reference: https://nixos.org/manual/nix/stable/command-ref/nix-shell