First, I take exaggellō (proclaim) to mean more literally, “message out.” Peter uses a different word than the typical “proclaim” (kēryssō) used elsewhere in the NT from which we get the concept of “preaching.” Exaggellō is made from two words ek (out) and aggellō (I message). The verb, aggellō is from the noun, messenger (aggelos, where we also get "angel" as in the angels were God’s messengers—you get that). So, Peter picks an atypical word so we are allowed to rethink “proclaim” a bit and take his choice a bit more targeted to the idea of “messenger” getting the message “out.”
Second, the word “that” (hopōs) that follows the wonderful list of church metaphors is also specific as a conjunction (more nuanced than the English "that" implies). The English “that” here gives the idea of goal . . . i.e., it is the goal or result of being “chosen race, a royal priesthood, a holy nation, a people for his own possession” to proclaim (which in English implies speaking, content, words). The word, hopōs, however, that Peter uses here is more nuanced and gives the sense of “in this manner.”
So, here is a fresh reading of this most wonderful verse:
"So that in this manner—being God’s own possession as a chosen race, a royal priesthood, a holy nation—you, the gathered-church, are to message the excellencies of him who called you out of darkness into his marvelous light."
Now, this is powerful reading of the text and underscores the significance and importance of the presence of the gathered-church in a community. Furthermore, this reading makes more sense of what is to follow regarding suffering as church, living in the Empire as a church (a people), and as a church made up of slaves, masters, wives, and husbands (1 Peter 2:11–17), which all ends with the church as God's apologetic in the world (3:17).