hi garrick,
to use the new interface for sharepoint library or list, you should enable the related settings as the following article mentions:
switch the default for document libraries from new or classic
in your situation, if you have clicked “x” to close the new look chosen window, as a temporary workaround, you can use another different browser or open the browser with pure mode like
inprivate browser, incognito window for chrome to open the library again, then you should see the following window again:

please let us know any updates when you are free, so we can continue helping you sort this issue out.
thanks,
kerwin