Does anyone have a complete list of words and theme categories? Currently trying to go off an old GameFAQS page that doesn't have all the Things words listed.

I don't know of one that exists. I honestly started keeping my own lists as I played and still to this day don't know all of the answers. Truth be told, I haven't encountered probably 50% of the Whatchamacallits mentioned in the GameFAQs list, so take it with a grain of salt.