I have written a series of posts on this subject of Neovim and Nix, that go from basic to more advanced. Here is the first. Basically, beyond just packaging the editor itself, Nix and/or Home Manager can replace the various Neovim plugin managers. With Nix, there is no need for LazyVim or Mason, for example. In terms of how to get started without something like Lunarvim, there are projects like kickstart.nvim and more simply, mini.nvim. For a really full-featured solution, you could try Nixvim.
If you will allow me to take off my Nix and put on my Neovim hat, I would like to encourage you to reconsider your overall approach: I actually “got good” at Vim and later Neovim when I put in the gradual time to build up the editor. For example, learn what C-X C-O does and why: then you will better understand what kinds of completion are available to you. Building up the editor will make it long-term easier to maintain and less buggy.