Though it seems impossible to colonize galaxy at sub-light speed but without FTL travel we can still colonise the universe at sub-light velocities[ using self replicating probes and Bioprograms which I’ve discussed recently], but the resulting colonies are separated from each other by the vastness of interstellar space. In the past trading empires have coped with time delays on commerce routes of the order of a few years at most. This suggests that economic zones would find it difficult to encompass more than one star system. Travelling beyond this would require significant re-orientation upon return, catching up with cultural changes etc. It’s unlikely people would routinely travel much beyond this and return.