Have tried this myself and never had much luck trying to install nvim plugins via nix.
I’ve found the best way is to just use the plugin managers built for neovim, I’m not sure if this applies to all of them but lazy.nvim seems to be fairly declarative anyway, have home-manager map a directory to .config/nvim/ and away you go
As a side note though I think it is rather silly just how many different neovim package managers there are, which at the end of the day all do the same thing in very similar ways