I think you're pretty much right. Everyone that was going to join had already joined. They only way to continue the expansion was through force of arms. And that was impractical, beyond a limited extent. We did get a bit more expansion by allowing the southerners to gain more influence within the church.
We squashed every other religion on the continent. The only ones that remained for quite some time were ineffectual, and nearly extinct, remnants and scraps. In that sense, SA did achieve a form of world domination. It may not have converted the entirety of the nobility, or replaced all realms with theocracies (neither of which were truly core goals of the religion anyway), but it did achieve religious domination of the island. Now it will live to carry on the legacy, and to form various splinter factions and related variants.
From an OOC perspective, the splinter/derivative faiths are the next obvious step in the story of SA, even though the IC/IG faith itself will not appreciate their existence.
