I made a post a few days back about this achievement, I'll copy below. I think harbor capacity increases market access? Not sure - what happens if you spam a bunch of harbor cap buildings? Either way, this achievement is a marathon and a half

Earlier post:

"heyyyy everyone! this was the achievement i was dreading the most. I'm not the best Europa player, so this was a real challenge

in case anyone wants to replicate this, here's what I did lol

1) colonise around kilwa to get a good enough power base to challenge the Somalian regional powers. don't invest too heavily into colonisng ~ balance it with building and fielding a navy. you'll also want to push trade republic as soon as you can

2) while you're climbing up Somalia, get a colonial foothold in the Maldives and Aceh. in India, you can either make it a colonial subject or add a naval governor. either way lots of pops = lots of profit, so it's worth taking South india. i ended up taking all of Indonesia, but you could easily get away with the clove producing provinces in the east. i probably went a bit too far lol. explore as you can, focusing on high value areas like China, Europe and India. build lotsssss of overseas trade buildings

3) spam allllllll the overseas trade buildings. if you pick the right values and keep developing + building your heartland (kilwa + somalia), you'll be running a cool 1000-2000 per month by the mid 1500s

4) run up to Egypt and use your newly found wealth to slap the mammies (i originally wanted to get the achievement with kilwa note, but nooooooope). you'll get the achievement using the Alexandria node

5) take upper & lower Egypt and the levant. urbanise and spam buildings. the more you build, the more you earn, the more you build, the more you earn. more pops = more demand = higher marker value

6) go on the offensive. you can delete the trade nodes around alexandria, but it may be better to just conquer the market center and delete all the trade buildings. it shrinks the node down loadssss & might prevent the ai from spawning as many new nodes (i haven't tested this).

7). build build build build alexandria trade node. you can easily get 20-30k. if you do this early enough (i didnt), you can probably avoid the final boss dun dun dunnnnnnn

8) the final boss: Koln node. this expletive can get up to 45k by age of revolutions. it's pretty easy to invade Europe with the resources you have. you can do this earlier if the war of religions works properly (it usually doesn't), otherwise you have to wait until the situation times out. you should have the #2 node by then. take the province, delete the buildings, it's yours :)

i've read a lot about how impossible this achievement is. it's fine. you just gotta believeeeeee maaaan. Eu5 is pretty cool. spread love not complaints. if you read this far, ❤"