Noting that the longer forms in psychonaut’s list are compounds (thereupon, themselves), I’d suggest that the reason most of the longer th- words in English begin with unvoiced th is that they were borrowed from Greek, either directly, as coinages, or via Latin, and initially began with theta, unvoiced th. Classic Greek did not have a voiced th phoneme, though in modern Greek delta has that sound. The Germanic/Scandinavian words tended to be short; the Greco-Latin ones a mix and more commonly long. So most longer th- words ended up with unvoiced th.