The Solomani strength was determination. No matter what the 3I did, the Solomani never gave up. Yeah, they should have many client states rim ward.
Any time we've seen a One Party State in the real world, they've always been 'forced' to have this continuous, ongoing "struggle" for (or against) X, Y, or Z, and by an amazing coincidence, the main side-effect of the struggle is to justify The Party and everything it stands for, and everything it does.
So yes, the Solomani Confederation will be very determined, for as long as SolSec and the Monitor Corps say they must.
Being Terran human, they would be anyway, but as presented the Solomani Cause seems destined to take its fair share and then some of the Solli's official efforts.
I have also always thought that there should be a whole lot of rimward expansion or client states - but maybe those would be a distraction from "the struggle" and are thus not allowed. If so, it was a very subtle decision by GDW, kudos.