Hey, AstroNvim core maintainer here! Just wanted to clear something up. AstroNvim is in no way based on LazyVim, AstroNvim existed for almost 2 years before LazyVim existed...
You can definitely put all of your configuration into a single file for AstroNvim if you want.
In the docs it shows the minimal configuration to get AstroNvim running which is <10 lines in your ~/.config/init.lua file and then anything else you can just drop in that same file if you want. (https://github.com/AstroNvim/AstroNvim?tab=readme-ov-file#mi...)