HungryForFood
Prince
Bonuses are related to trade routes because of Victoria's UA, which lets you send gold trade routes to yourself, so both the starting and destination city gets the bonus.I think it's too much emphasis on trade routes and it wouldn't really change much. How many trade routes can you even have in industrial era, how many of the statecraft, petra, colossus and sea WC resolution with the wonder did you get? This seems pretty boring, by that point those yields are going to be hard to spot. I could see something like "originate all TRs to capital/send them out of it to get a slight help with wonders" as this way it'd be a noticeable 20~ Prod there, but it's not convincing. Better than what it is now, but if it was given on top of a slight base yield increase, it'd be fine I guess.
If you think an effect is weak, you can always increase its strength, so the question you should be asking is: is this bonus interesting or fun?
I suggested +2 production per trade route, scaling with era. So, when Steam Mills are unlocked, that would be ~8-10 production per trade route. Alternatively, perhaps no era scaling but a larger value would work, since number of trade routes already scales with era.
Another possibility is to grant a modifier instead of just raw production. Perhaps 5% per trade route, capping at 50%.