How can I programmatically insert content via a script or import instead of through the UI?
The most direct answer to your question is Migrate Source XSL module for the D8 Migrate API.

Or hell, dont have Windows/Excel installed do the same with Google Sheets Migrate Plus plugin.

I'm not sure if there is a UI for Migrate yet in D8. But this would be the most flexible solution. If needed you could use the drupal api itself to code simple insertion from a php script to create Nodes from an excel script; but the above migrate plugins should be much for flexible to debug in the long run -- but may require Drush support currently.

Edit

Ask previously, how to create a Node in D8:
Programmatically create nodes