How can I programmatically insert content via a script or import instead of through the UI? I have a custom content type and I now need to create thousands of entries using that type. It would take a very, very long time to go through the interface and add these one by one. I'm hoping there is a plugin, module, or some kind of API so that I can create all of these entries in an excel and simply import them into the database. Is this possible?

I have not been able to get Drush working so that isn't really an option for me.
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