Best ways to make gold alone.

Just sail into the wind looking for sunken ships and treasures on beaches. Try to stay moving a lot.

Merchant quests,

Pull the voyage, if no golds, steal crates and cancel. Only grab gold and black animals, gun powder barrels and cannon ball crates.

Always check ship wrecks and get a map app for your phone, points of interest on islands often have chest spawns.

Turn in frequently and only sell your animals if you have quests for them ( worth approx 10x the regular sale price).

Don't get tangled up in skull forts. They are time drains most of the time.

Keep your head on a swivel and sail against the wind when being persued tacking to the wind often. Few players bother with the finer arts of sailing and it has saved me often.

Lastly. Keep snake crates on the back of your sloop, on the canvis roof. Alot of galleons just leave me alone as they see I'm a solo sloop doing merchant quests.
(Not having the voyages for the animals makes them less attractive).

Stay away from pigs if you can. Waste of bananas.