I had the impression that there was little Solomani colonisation rimward of the Near Bootes Cluster all through the Interstellar Wars, Rule of Man, and Long Night periods. Serious colonisation of Canopus, Aldebaran, Neworld (and beyond) didn't happen until after the establishment of the Autonomous Region ... or even until after the Rim War. (Parallels can be drawn with the Soviet Union's attempts to strengthen their fallback position by building industrial centres in Siberia.)
Terran settlements in Magyar though are _very_ early, and plenty of "bolthole colonies" were established during the Interstellar Wars. The Aslan didn't "develop" jump drive until 300 years later, by which time the Solomani had colonised within a subsector or so of Kusyu. So path of least resistance is going to be anywhere other than trailing.
But yes, your point is taken. Authorial fiat for the purposes of narrative excitement is a much better explanation of low-probability events like the fall of Glisten, the Mexican annexation of the US Southwest in 2300 AD, and the fact that a previously independent TV celebrity and a previously independent socialist are two serious contenders for US major party presidential nominations!