Even before you go down to the actual implementation on how fuel is represented and managed inside the ship "objects", this is a new feature altogether.
An "on/off" button for a non-powered optional. Fuel tanks, cabins, don't show up in modules. Cargo racks do show up but there's no "off" button. If you now present Fuel tanks, that's UI work for the tab, any text added there has to be approved, go through localization to other languages, etc.
THEN the ship object needs to be able to update its max fuel state (regardless how is actually implemented) outside of outfitting, that's exposing other properties/methods, etc. Which is what you refer "operating fuel capacity", another feature/property that doesn't exist (I assume doesn't exist, and its state can't be changed other than through outfitting)
We all have to agree, when it comes to requesting features on code that we don't know, we can't just make assumptions that is easy, or trivial, or even super complicated either.
Cargo racks work similar to fuel tanks, you add them to the ship and is a flat currentCargo = currentCargo + newCargoRack;
To finalize, I did hit like on your suggestion, would be nice. But reiterate doubt it has any significant QOL improvements, plus the more I think about it, the more I believe is not trivial change at all.