Doing some research on Sarna it appears that Avalon City wasn't founded until the 26th century, when it was adapted from of all things a theme park. This begs the question, what was the capital before that? Is it mentioned anywhere?
Per the OG Davion sourcebook, Avalon City proper was founded by Lucien Davion in the 24th century, and completed in the late 2390s.
But then there are
other references in the same book (in the section about Katherine Anne Davion) that mention her being an Avalon City alderman in the 2230s-2240s timeframe as well, and that it was her policies that caused Avalon City to rapidly grow into the most cosmopolitan city not just on New Avalon but in the local stellar region, which in turn was one of the reasons why it was chosen as the capital of the FedSuns.
Finally, there is mention of an early settlement on New Avalon named Ingram, which in 2253 was the second-largest settlement on New Avalon with a population of five thousand people... until the growing Avalon City poached most of its population away.
I know none of this directly answers your question, but I thought it might at least give a little insight.