In 1766, he was a chief negotiator on behalf of his brother, King Solomon I, who had been driven out of his capital in an Ottoman-supported coup in favor of his cousin, Teimuraz of Imereti.

Rostom dispatched his brother Besarion, Catholicos of Abkhazia and, hence, the head of an Orthodox church in western Georgia, to the Ottoman pasha in Akhaltsikhe to ask aid against Solomon.

Solomon’s diplomacy won and Besarion was cast in prison in Akhaltsikhe, but he escaped to the Principality of Mingrelia and declared himself "catholicos" of the local Orthodox church.

In the years that followed, Joseph pushed for efforts to persuade the Mingrelian government to abolish the rival catholicate established by Rostom's fugitive brother Besarion.

A new wall was constructed around it; several new estates, serf households, precious liturgical items, and a collection of manuscripts, including the 13th-century Vani Gospels, were donated to it.