Unfortunately, I wouldn't say I'm particularly well informed on this topic so won't be of much use
Theology for Greeks like Aristotle was something a bit different from Abrahamic theology and was part of philosophy similar to metaphysics. It was intricate, but in a different way and with a different purpose. It may have got a bit more 'religious' by the time of Neoplatonism, which in turn influenced Abrahamic theology.
As for people dedicating themselves to one god, I can't say much, but priests of Cybele used to cut their balls off which seems quite devoted to me
"Attis" may have been a name or title of Cybele's priests or priest-kings in ancient Phrygia.[115] Most myths of the deified Attis present him as founder of Cybele's Galli priesthood but in Servius' account, written during the Roman Imperial era, Attis castrates a king to escape his unwanted sexual attentions, and is castrated in turn by the dying king. Cybele's priests find Attis at the base of a pine tree; he dies and they bury him, emasculate themselves in his memory, and celebrate him in their rites to the goddess. This account might attempt to explain the nature, origin and structure of Pessinus' theocracy.[116] A Hellenistic poet refers to Cybele's priests in the feminine, as Gallai.[117] The Roman poet Catullus refers to Attis in the masculine until his emasculation, and in the feminine thereafter.[118] Various Roman sources refer to the Galli as a middle or third gender (medium genus or tertium sexus).[119] The Galli's voluntary emasculation in service of the goddess was thought to give them powers of prophecy.[120]