This is inherently a Hard Problem™, since completions may require evaluating arbitrary derivations (e.g. building a custom Linux kernel).
For "what symbols are available", the nil LSP implementation[1] works for anything in scope that doesn't require evaluation. It also includes completions for the stdlib and NixOS options (in certain contexts).
Another LSP implementation is nixd[2], which is trying to tackle the problem of evaluations for completion.
Maybe then IDE is not the best target, but rather a clear "REPL-friendliness at all times" is, e.g. everything is built around constant re-evaluation where you can discover things where interested, maybe put stop gaps and prints easily.
For "what symbols are available", the nil LSP implementation[1] works for anything in scope that doesn't require evaluation. It also includes completions for the stdlib and NixOS options (in certain contexts).
Another LSP implementation is nixd[2], which is trying to tackle the problem of evaluations for completion.
[1] https://github.com/oxalica/nil/
[2] https://github.com/nix-community/nixd