If it's just supposed to be a cargo hauler, then yes, there are too many weapon mounts. A Tarsus (noted for durability and hazardous-area performance) has just four small ballistic mounts; a Colossus has the same number on a larger hull, and an Atlas has fewer at three. Eight small and two medium is a lot more (potential) firepower.
As to the idea of a "strap oversized cargo on the outside" transporter... it depends on how "hard" you want the physics to be. Starsector, I feel, sits closer to the "hard" end than the "soft" end; in service of that, the ships' mass is (nominally) balanced on their engine plumes, as if they were actual rockets. So if you imagine your sprite as a flat plate, with the engines at one edge, onto which you strap cargo onto the flat side... all of a sudden, those engines aren't pushing against the center of gravity, they're offset.
In my opinion, if you want this to be a "haul oversized cargo" sort of affair... then I'd delete roughly the middle fifth of the ship, see attached image. This allows the oversized cargo to be "balanced" on the cylindrical support near the stern, extend dorsally and ventrally at will, clamped port and starboard by the six yellow box-things, and covered (somewhat) by the port and starboard wings. If you were thinking of cargo oversized to the point of being larger than the ship, well... at that point, I think the transportation corporation would be thinking more of "tugs" rather than "flatbeds".
ETA: Alternatively, you can read up on how David designed the Shepherd,
part 1 and
part 2, and then think about how you'd apply that sort of thinking to your Space Hauler.