Just to try to bring this back on track...
A clarification on your initial question because maybe it's unclear.
Are you asking that as your ID states you can fly a ship with a hold of 3800, why aren't there any that you can fly?
Why therefore was 3800 stipulated knowing that there are no ships that fit that description?
As there aren't any, why was it put on the ID in the first place?
Are we expecting any new designs to fulfill this role in the near future?
I'm asking this so that I understand the perspective of the question, rather than the implication which may have been misinterpreted.
(That aside, the Border Worlds transport will probably be your best bet as everything currently stands).