Ok heres some trade tips. I have posted this several times you know, latest in the tips and tricks post.
First, never keep a merchant in your home mode. The bonus is negligeable 70% of the time. The situations that call for it are SUPER rare and mostly involve being unable to send them anywhere else.
Second, only steer trade if it is in a neighboring node to your home node. Otherwise, collect. The exceptions to this are mostly the cape of africa and colonial nations, but even then you are usually better served collecting unless you UTTERLY dominate one of the european end nodes. I mean like 70-80 percent control or more.
Third, know when your beaten. Dont try to move in on sevilla if you dont own any provinces there, it has 5 bloody super trade provinces (the ones with great bonuses). If you cant get at least 20% of the power, go somewhere you can. Of course, if this sevilla trade node has 100 gold in it, that 5 percent you can eek out is actually worth it.
Fourth, each merchant gives 5 naval forcelimit, unless that changed recently. This ties into protecting trade. Super worth it, but with diminishing returns. I suggest trade fleets of 10 light ships, that you combine if you have multiple sets in one node. This keeps things nice and tidy.
Fifth, experiment. It takes time to optimize merchants, and no two games are the same. Start messing around and watch the trade income tab.
Some other tips. Trade buildings are bloody spectacular. Marketplaces are surprisingly meh, but the rest are mind blowing. If you have admirals but they arent doing anything at the moment, stick them on a trade fleet. Super valuable, but not quite worth the leader slot. Merchant republics should stack thier trade posts on existing center of trade bonus provinces. The stacking bonuses are super duper.
And lastly, and maybe most importantly, trade wars. This are tricky because they can EASILY backfire. you start this babies with the covert tab in diplomacy, fabricating something or another that I haven't bothered to really read. Declare war, blockade ports, and then force them to transfer trade power and maybe war reperations if you do really well. Do not underestimate the sheer amount of bank you can get from trade warring everyone.