Thx for trying the TP change out, Thal!
It is very interesting that those adjacency modifiers are possible
Let's try to think this through systematically:
1) Is the current amount of available gold (empire-wide) ok?
I think there's a fine balance between making things like CS alliances to easy to get (too much gold) and reducing choices (only enough gold to pay upkeeps). Overall, the current overall amount of gold might be roughly ok.
2) Should TPs be buffed in general?
Much of Thal's modding was to make dumb trading post spam less viable, so buffing them could counteract this. Other improvements are buffed through lategame techs (fertilizer), but TPs are buffed through policies. Overall, again no need to buff them.
Conclusion 1: TPs should still have 2 yield on average.
3) What other problems are to be considered?
- Rivers/Lakes only boost farms, making them the ideal choice very (too?) often.
- Riverside tiles are extremely strong in vanilla.
- Coastal regions are not as valuable as they are in real life (or even inferior to landlocked regions).
- Trading posts are boring gap-fillers, because they are equally valuable everywhere, their placement doesn't matter.
- TPs are a weird abomination without RL counterpart. Seeing them as villages increases immersion (especially with a graphics mod)
4) Is there a desirable way to change TP yields?
Suggestion A - small change
TPs are still 2 base yield
Rivers don't give any boost from start.
Civil Service makes riverside farms more valuable (as before).
Currency makes riverside TPs better.
To compensate the lost gold from rivers, coastal adjacency could boost TPs, too.
Suggestion B - larger change
TPs get only one base yield
Rivers don't give any boost from start.
Civil Service makes riverside farms more valuable (+1

as before).
Currency makes riverside TPs better (+1

).
To compensate the lost gold from rivers, coastal adjacency could boost TPs, too.
To compensate the 1 base yield, TPs adjacent to a city would give +1

.
5) Possible problems:
Suggestion A would probably mean a slight overall increase in available gold, Suggestion B would probably reduce the available gold somewhat. [EDIT: Thal showed that the yield boosts are cumulative. So in variant "B", TPs next to rivers AND cities would have 4 yield, probably resulting in vanilla yields - 2 average - overall)
The reduced yield from rivers in the very early game could mean it's tougher to get enough gold at this stage. [EDIT: Thal suggested boosting gold from the palace to compensate]
6) Advantages:
- Less river dominance
- Coastal regions buffed
- More interesting and realistic TP placement
- Probably understandable for the AI.