I don’t have the skills to illustrate it, but I would like it to be more ovoid shape.
I understand, but that should steal some space. Generally I would hide the fuel tanks in the curvature. The formal deck plans should then be a bit smaller than the ship tonnage.
Economics are a hard one. The ruthless drive to make ships as profitable as possible make for narratively boring ships. The focus here was on making a different vessel. You don’t see many modular ones. I wanted a ship that could swap from module to module and take on different missions.
If the Kestrel started out as a military or scout design, economics would not be such a big concern.
Even the massive bureaucracies of the Navy and Scouts care about budgets, but if that doesn't matter to your game...
We can skimp a bit without defanging functionality completely.
Interesting you mention a 30-dTon module. That is where I started, with a 100-dTon ship and 30-dTon modules. Each of them had to be custom made. Robert Pearce has done a huge amount of work with his geomorph series of deck plans. He has 50-dTon quarter modules. It seem foolish to create my own 30-dTon custom ones when I could upsize the mothership and use this huge collection of ready-made modules.
I would use a bigger ship for 50 Dt modules, perhaps two modules in a 200 Dt ship, or four in a 400 Dt hull?
Several modules allow much more flexibility, perhaps one habitation module and a mission module?
I might make something like this (sorry, I'm a bit obsessed with flexibility and efficiency):
200 Dt light streamlined hull with aerofins and radiation shielding. Minimum TL for J-2.
Budget J-2 & M-2 drives with just enough power for either thrust or jump.
Two 50 Dt modules & five 1 Dt modules for better sensors, docking clamps, repair drones, batteries, etc.
Cheap at MCr 46 in quantity, noticeably cheaper than the 150 Dt design.

Can be operated by a single person in cramped quarters, with a lot of software support...
Normally one module would contain more crew space.
J-1 (20 Dt) worth of fuel in fuel/cargo container, so can be used as cargo space when not doing J-2.
J-1 (20 Dt) worth of fuel in demountable tanks, can be removed for another 20 Dt cargo.
Drop tank mounts for two 40 Dt drop tanks.
Collapsible tanks for a bit of extra range, if free cargo space is available.
Can carry a few external pods or smallcraft in modular docking clamps, reducing performance.
So, can operate as a ~300 Dt 2×J-1 ship with close to 200 Dt payload...
m/10bis computer with a m/5bis/fib backup, so can run J-2 and Virtual Crew at the same time.
If you are a bit less obsessed about cost, make it 250 Dt and have decent crew space, still under MCr 60:
OK, that is much bigger than the original design, but not much more expensive, and much more capable.