Can I create sub-labels to recreate/maintain the folder hierarchy I set up in Thunderbird?

Yes, you can create “nested labels” in Bates Gmail so you can build a label hierarchy, just like you can with folders. When creating a new label, just choose another label under which to nest it.

Note that labels allow you to categorize messages in multiple ways, so you’ll probably find that you don’t really need a lot of nested labels. For example, you could apply the labels “Short Term,” “Alumni,” and “Lewiston” all to the same message. You can then find the message by clicking any of those three labels in your Labels list. All three labels appear at the top of the message, so you can always see how you’ve classified it.