Put the following in your ~/.zshrc

for script in ~/.zshrc.d/*; do
  # skip non-executable snippets
  [ -x "$script" ] || continue

  # execute $script in the context of the current shell
  . $script
done

All executable scripts in ~/.zshrc.d/ folder will get executed. The scripts are execute is lexicographical order so putting 10_, 20_ etc. in front will run them first.

If you have a huge ~/.zshrc file you can start copying parts into something more organized.