καὶ διὰ δὴ ταῦτα πάντα τὰ παθήματα νῦν κατ᾽ ἀρχάς τε ἄνους ψυχὴ γίγνεται τὸ πρῶτον
i think one of them, κατ᾽ ἀρχάς τὸ πρῶτον , must be redundant.
maybe he wants to say something like "Now and at the beginning, the soul is first bereft of mind when bound within the mortal body’. But this ‘now and at the beginning’ is obscure.
τὸ πρώτον ὅταν should be taken together, “the first time that it’s bound to a mortal body” (i.e. “as soon as …”) and this picks up κατ’αρχάς in the νῦν κατ’αρχάς τε phrase.