Four ships is a bit cheap for starting a mod. Also i think the most important part in Starsector's modding is the spriting (unless you are a coding wizard but that's a whole different thing) and i think yours can definitely be improved. I borrowed one of your ship and modified it to show some trick.
First of all, your sprites feel a bit flat and the best way to avoid this is to add shadows. The easiest way to do this is do outline some parts of the ship with the pen tool with the black color on a new layer, then reducing the opacity to 30%/40% (i'm assuming you're using Photoshop or at least Gimp). It's good to avoid a flat sprite.
Also plates armor are rarely perfectly clean (unless you're trying a high-tech, Tri-tachyon style but in my experience it's way harder), greebling is the best way to add details to avoid having a ship too "clean". Here i copy/pasted some textures from the Eagle cruiser and lowered the layer opacity to 10%. You can also toy with the layer types like luminosity, saturation etc to try new effects.
And finally i copy/pasted some tiny parts from vanilla ships on yours. Same thing as above, lowering opacity, placing layer type on luminosity, using the eraser tool to get rid of the unwanted pixels and that's it! When you start spriting, lot of greebling can help "filling" the ship. At least that's how i started.
Also if you want to make a mod, there's always a cheaty way to get people playing it, capital ships. No matter how broken, badly designed and lore-breaking those are, everyone love trying giant ships. It's kinda like cheating but it work.
I mean, who wouldn't try something like this?
