Build smaller cheaper transports, that way you get the flexibility to get the right amount of payload at the right time without wasting ships or running half-full ships.
That was quite a lot of house rules at the same time. At around GCr 10 it is quite expensive, especially for its limited cargo capacity.
This is actually a regulation LBB5 design that can be built at TL-10:
Using LBB2 Z drives it has Jump-4 and Man-4 with 1200 + 100 Dt cargo. MCr 600 as a standard design.
Code:
QT-C644442-000000-00000-0 MCr 594 3 000 Dton
bearing Crew=19
batteries TL=10
Cargo=1201 Fuel=60 EP=108 Agility=3 DropT=1200
Dual Occupancy 1 201 739
USP # Dton Cost
Hull, Streamlined Custom A 1 800
Configuration Flattened Sphe 6 144
Scoops Streamlined 2
Drop Tanks 1 200 Dton 1
Total tonnage 3 000 Dton
Jump Drive Z 4 1 125 240
Manoeuvre D Z 4 1 47 96
Power Plant Z 4 1 73 192
Fuel, #J, #weeks J-4, 4 weeks 4 60
Purifier 1 50 0
Bridge 1 36 9
Computer m/4 4 1 4 30
Staterooms 16 64 8
Cargo 1 201
Collapsible Tanks 1 300 Dton 1 13 1
Bay Empty 1 100 1
Gig 20 Dton 1 26 15
Nominal Cost MCr 738,94 Sum: 1 201 739
Class Cost MCr 152,03 Valid ≥0 ≥0
Ship Cost MCr 594,15
Crew & High 0 Crew Bridge 10
Passengers Mid 0 19 Engineers 3
Low 0 Gunners 0
Extra SR 0 Frozen Service 6
# Frozen W 0 0 Flight 0
Marines 0 Marines 0
Note that it costs about a quarter of your ship per Dton cargo. It also uses less crew per Dton cargo, even with a regulation crew.
It is cheap enough to be permanently attached to a destroyer, or shuttle back and forth to a larger formation. The empty bay allows it to carry a variety of cargo or small craft, including standard Imperial 50 Dt craft, such as the Cutter, or even a Heavy Fighter.
With 16 staterooms for 19 crew it has plenty of space for gunners or supernumeraries.
Filling the collapsible tanks in the cargo hold it can do two jump-4 without refuelling, still carrying 100 Dt cargo in the empty bay. That would e.g. take it from Regina to beleaguered Jewell with some crucial cargo and passengers.
Build it at TL-12 with a m/6 computer and it can even do J-6 w/o external tanks. Quite usable as a naval scout or courier.