I understand this isn't the question of the post, but you seem to have a misconception here about your economic capital which is pretty common.
The planet you have your economic capital on certainly doesn't have to have a starport, but to say "of course it lacks one" makes me wonder what you think about the planet this is on.
Sorry for that, "of course" only within my prefered game style. I am by far no power player and I don't tweak the best out of my planets, but lacking information how things exactly work I prefer to build up my economic capital at a planet with a long distance for trade routes, a high population and lots of stock markets and the economic capital. Mixing all those possible tarde income factors up took the space for my starport away, it hurts to have one there just for freighters (that build for ages btw).
Unlike the research and manufacturing capitals, which multiply the output from their respective planetary structurtes, the economic capital ONLY boosts the planet's tax income. It does not add anything to any other economic structures on that planet.
But how is that bonus different from other economic structures like stock markets? All I want is to maximize the planets tax income. Then I usually establish trade routes as far away as possible, taking the target plantet's income into decision.
Unfortunately I am no native speaker, so did I understand you right? You are claiming that I get more out of the economic capital bonus if I heavilly increase population in that world and therefore loose some stock markets and building morale structures instead?
-- A single 10 parsecs long journey which takes only four turns or less (ship engines, baby) to complete the deal, in theory, would start at 3bc... as with a 50 parsecs much longer trip grabs a whole 20+ turns to stack (on average) 6 to 10 Bcs per "completed" deal.
Now this is the first time I read about a "completed deal". I always thougt you get income from your trade routes every turn depending on your frighters distance from your home world. Freighters close to their home world add less to yout total trade income than those far away. Very short trade routes wouldn't make much sense then...
-- Calculate all you want... shorter routes are really the most optimal flow of cash and less likely to be 'flipped_off_war_risky_random_conditions' and cancelled altogether. The natural growth of such routes can only last for so long and should be as much efficient & protected as possible.
On the other hand I also don't know how the natural growth works, so it is possible that at some point those short routes also get very lucrative. Usually my trade routes have a long life, but if this natural growth doesn't max out too early I would rethink my strategie.