You ask whether I would have become Orthodox even if I’d lived in the tenth century “when there were no fully developed forms of mystical practice”. I’m afraid you picked the wrong century if your aim was to show the absence of Hesychasm at earlier periods, for St Symeon the New Theologian, who lived from 949 to 1022, is one of the most important and most often referenced authorities in the entire tradition.

Your write that “many of [this tradition’s] specific practices are not attested in earlier centuries”. The operative word is “attested”. If by this mean you to say that explicit written descriptions of Hesychastic technique are harder to come by in the extant texts of earlier periods, then of course you’re right. But this proves very little. Like all forms of genuine esoterism, Hesychasm has always figured largely as an oral transmission, with only the briefest of hints being put into writing. This was true even in the fourteenth century, when there was a certain flowering of Hesychast spirituality in connection with the two saintly Gregories (Gregory Palamas and Gregory of Sinai), and it remains true to this day on the Holy Mountain.

Admittedly, as one moves forward in history an increasing degree of specificity can be found in the surviving written records, but the essentials of Hesychasm—the superintendence, direction, and extension of attentional energy by means of the sacred power of the Name—are nonetheless clearly present, if one knows what to look for, in the teachings of several of the very earliest of Christian masters.