If I understand it right, confluence works to increase rate of culture spread, meaning you get improved allegiance on your planets faster, meaning you get increased tax rate income faster.
I don't remember which one is confluence off the top of my head, but one of the culture modifiers, spread rate, is how fast the culture spreads (there is some debate if this has any affect if hostile culture is in your way). The other affects how fast allegiance changes once culture reaches the planet (0.07% is the default max decrease, 0.14 the default increase).
In theory, both will lead to your planets getting the 10% allegiance bonus a bit faster, though I'd doubt it would be enough to pay for itself in pure economic terms. Keep in mind even your homeworld gives what, 20-25 credits per second? 10% is about 2 credits per second plus some metal and crystal, but that's only going to be earned on your culture center, and other planets won't give anywhere near that much.
plus at least two tradeports
Tradeport will make money even with just one trade port, as weird as it sounds. Its just it gets absolutely no multiplier from trade route.
There's also the fact that culture gives other bonuses and forces the enemy to respond, otherwise impairing their economy and expansion. My guess is that tradeports must result in better credit income eventually, even just 2, but the upfront costs of them is daunting as advent when I would prefer to put that into culture and ships for military might and more planets colonized.
The big thing that leads to trade being so widely use at the expense of other civic options is that is scales infinity. Unless you sabotage a very long trade chain, each trade port will increase you income at least linearly, there is no diminishing returns to negate their payoffs. Culture and refineries on the other hand have limits (the max culture cap at 10% without upgrades, or the 3/4 refineries each resource asteroid can support) that make building multiple of them at a planet less attractive or useless. The only thing culture has going for it is that it can affect multiple planets from one gravity well. Every other culture center you build will give no where near the benefit of the first (unless its in an area so remote that your culture cannot possible reach it).
There's also the fact that culture gives other bonuses and forces the enemy to respond, otherwise impairing their economy and expansion.
If you go culture early, this is why you should do it. If you're not trying to force them to leave a captialship behind or forcing them to get culture as well, I'd say after 3 tradeports/culture centers the trade will start to win easily. Going an entire game without trade is probably suicide, it just can't scale well enough to provide the raw income trade does mid game, let alone late game.