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.