@pumpa-cat There is no safe way to gather that amount of loot unmolested, (thankfully or the cheese would worse than it is, especially in Alliance servers).
But the quickest way to get loot fast is Lost Shipments, Gold Hoarder vaults, and Sea Fortresses. All have little PvE interference (or easily handled like in the Fortresses, you can literally pick them off from the entrance as they come up/down the stairs with a single pistol shot each) and can be completed pretty quickly for good loot. You can also fly an Emissary flag while doing these, but that increases the chances you will be attacked. Also doing these during the Gold Rush Hours will double your haul.
Also doing voyages in the Devil's Roar will grant better loot, but you have the added danger of volcanos and geysers. But a lot of players will avoid that area because of that.
Also in the Maiden Voyage there are several hidden caches of coins and doubloons around that island, around 20K if I remember right. If you did not get all them when you first played it, fire that up again and explore around for keys and hidden chambers. But once you find them all, they won't respawn so you can't cheese it for all eternity.
I hauled in about 150K in a little over an hour by doing 2 Sea Fortresses and a couple of shipwrecks along the way (I believe there was a skeleton sloop involved as well, but might be wrong on that) while flying a Reaper's Bones Emissary flag sailing solo. And this was with turning about half of it in when I was only Grade III, and part after I had already lowered my flag when I thought I was going to quit but spied a wreck close by.