IMHO, I agree with both OP and
@Daetrin post on trade, but here's my take on it
Civilian Access This is the fundamental key of trade: Civilian access. If your empire does not have civilian access, don't expect to make trade links. I think the discussions regarding happiness and ethos spread should not be based on trade, but more on the civilian access. Regarding migration. I agree with
@Daetrin's post.
Basis of Trade Following DD on Planet's resources, assuming no change was done on this mechanic,
The Basic Resources
View attachment 142998 Food is a requirement for Pops to grow. If there is plenty of Food, Pops will grow faster. If there is a lack of Food, Pops will be unhappy.
View attachment 142997 Minerals are used to produce most things in the game. If Minerals represent matter, Energy Credits represent work.
View attachment 142996 Energy Credits represent all liquid assets and energy produced by our Empire. Actions, such as clearing tiles, cost Energy Credits to perform. This resource is mainly used for upkeep, and although it can be hoarded, that might not be the best way of handling it.
Then I think trade should only occur to exchange food/minerals with energy credits, energy credits being the main "currency" in this game. However, if we disregards this basic resources as the "essential resources the empire controls", being civilian resources are treated separately, then the next mechanic wouldn't make much sense which is
Resource Modifiers There are two viewpoints on this actually.
1. Either an increase in energy credits and decrease in the amount of minerals/food produced for the nations selling minerals/food and a decrease in energy credits and increase in the amount of minerals/food produced for the nations buying, or
2. A gradual increase in the resources available to the empire, reflecting the overall gain of trade in the empire (when your empire opens up, automatically your supply of a particular resources increases as well). This is line with OP suggestion that as time grows and trade grows stronger, the modifier will increase up to a certain maximum where the two empires' economy becomes closely integrated.
Planet Tile Buildings or Stations We should try to add additional planet tile buildings or stations to make trade steering more sense. For example, with the creation of a CBD in a particular planet, other species prefer to go trade in this particular planet and increases immigration attractiveness. It will also help boost the production of resources in that particular planet, making it useful to be placed in large or huge planets.
Benefits & Loss for Trader-type Empires
If we decide to create a benefit for trader-type empires, then my suggestion would be to reduce the influence maintenance for these links while increasing the gains they receive from the empires, but they should have a negative resource production modifier to reflect their need to trade.
But we have trading via Diplomacy
While this mechanic is already in-game, this seems more like direct exchange of resources between one empire and another, while the discussion is more on civilian trade. Of course more strategic resources like the terraforming gases cannot be traded efficiently, but as the viewpoint suggests, creating an unknown unlimited resource modifiers is more useful, and if you can compound this with trades from other empires, it can be quite a boom for the empire conducting trade.