Is the title of tathagata ever literal? I think it translates as e.g. "one who has thus gone".

My question is whether any historical being has ever really and in entirety "thus gone".

Perhaps in spirit, or in expression, but if e.g. Gautama really did - wouldn't we all be likewise "thus come"?
The term Tathagata is gramatically ambiguous. You can parse it in two main ways. You can parse it as Tatha+gata, "He who has gone thus" which refers to going over to the other shore, which is a title of Nirvana, or you can take it as Tatha+agata (in which case the first a in agata drops out due to Sandhi) which means "He who has come thus" referring to coming to see the truth, or as "He who has come to see thus" because the term thus here is related to the term Tathata, meaning suchness, which is another title of Nirvana.