I've a non-canon write up on the Sung at the 2300AD Collective.
https://sites.google.com/site/2300adcollective2/contact-us/the-sung
I assumed, inspire by Robert Forward's writings, that the Sung made use of laser arrays to propel their solar sails. That would have given them reasonable transit times.
On the subject of reasonable transit times, one thing that occurs to me is that, maybe, the range of destinations in the Sung home system is constrained and the distances involved are compact. Stark is one obvious anchor for Sung interplanetary civilization, as is Home of the Mother, a garden world also in the life zone. These two worlds are in the relatively compact habitable zone of an orange dwarf star, though. The distance between the Stark and Xiang homeworlds, even at maximum separation, would have to be substantially less than (say) the Earth-Mars system.
Where else would the Sung go? Inner-system worlds would seem more likely targets for a Sung presence than outer-system worlds, at least going by the precedent of pre-stutterwarp Sol (colonies on Mercury, Luna, and Mars, but only scattered science stations in the gas giants). The rare trips to the outer DM+4 123 system might take a long time, sure, but they might be rare. For inner-system trips unassisted ion drives might well be enough.