In other letters of Gregory, Cyriacus is referred to as either servus Dei or abba. The first title suggests a monk, a latter could apply to both a monastic superior or a presbyter in charge of some church. That Gregory writes of increasing Cyriacus presbyterium may suggest that Cyriacus was indeed a presbyter, but the term itself seems to apply generally to money given to clergy, sometimes in terms of occasional donation, sometimes in terms of salary. Gregory also mentions presbyterium in letters 2.9  and 5.27 [XXXXXX].
It is impossible to determine whether the allusion to locus superior means that Cyriacus was already a defensor and was raised to a higher position than other defensores, or that he assumed a higher position, among other defensores, although he was technically not one of them.
For the high position of defensor in the society at the time, see e.g. Gregory's Dialogues 2.19.
It is important to note that Gregory calls Cyriacus his brother, and not his son, indicating some kind of familiarity or respect (or both). Usually, Gregory reserves this title only for other bishops.
See the analysis of this passus in Wiśniewski 2019: 330–331, in which he proposed, for example, the rendering of polypticus into "pay-list", rather than Martyn's "account book". Wiśniewski also points out to the importance of the public character of Cyriacus' elevation.
PCBE Italie 1: Cyriacus 6 proposes that polypticus should be understood in this context as an account book of the Gregory's Sicilian patrimony.