So many useful ideas here:
Re: his having been a crew member aboard the Coral Curse, but having survived the Yellow Fever epidemic and now having the attacking french captain as an enemy:
I actually think (3) is a pretty compelling backstory, and would definitely fill a niche no other PC in the party has filled, whether current PCs or retired PCs. And the timing would work well, because it seems likely the party will speak to some of Van Djik's former crew soon-ish...and you can follow the recent posts which will deal with revealing the mystery of what befell The Coral Curse, which would be info. your PC knows (at least in part) having been a crew member.
Currently the party includes:
Katerina del Corazon (Rogue 3, Fighter 1)
Old Zef (Wizard 4)
Nia Steeleyes (Ranger 1, Druid 3)
Etienne Rougeau (Bard 4)
Blaise Arceneau (Paladin 4)
So it's well balanced, and your sorcerer or warlock concept would fit just fine.
No need to worry about "what if I had spell X handy" because (a) The Coral Curse has been beached for several years, so it's possible your PC was lower experience level or even un-classed then, and (b) there's a "big curse" involved, so surviving might not just have been a matter of making good saving throws (though that certainly could be one route to survival).
Hmm. Captain Van Djik survived this long, and apparently others did as well. The Capt. is a dwarf, having resistance to poison damage and Adv. on saves vs. poison; but not having the same for diseases. Now he's about to succumb after these years. . . .
I think I should work on other parts of the story for bonds and relationships and fortunes before circling back around to the question of how this guy survived the epidemic. If there were Divine/Spiritual intervention, that would seem to indicate that this character offers (or offered) sacrifices to spirits, or prayed really devoutly to Bondye ("Bon Dieu"). The retired Doctor Hawken Varlock had a loa patron, so other people in the region could easily do the same. "Reason for survival" may be a bridge to cross another day.
Interesting notions here. I had some thoughts:
1) Redemption - What if, instead of being on one of the failed attempts to find the Gloriosa, he was actually one of the original sailors on it? One of the ones who pillaged and ransacked and made off with the treasures? Somehow...perhaps by dark pacts or sincere promises to higher powers...he survived its sinking, and now seeks the wreckage so he can try to make amends by returning some object of special value to its rightful owner.
Which might put him at odds with a greedy crew at some point, but that'll be interesting.
That's certainly something I had not even considered. Were there any survivors of La Gloriosa? Did it reach some distant shore
in extremis without sinking? Did The Kraken grab it whole and drag it down to the Abyss before anybody had a chance to jump off? Did the seas become strange while La Gloriosa was under cannon fire from pursuers, with parts of the poop deck blown clean off the superstructure, only to fall to the sea with the still-living bodies of sailors cast like puppets into the air to fly clear of the flotsam? Did the flotsam's fall to the water, and temporarily under it, extinguish any flames, so survivors could swim to and cling to (or mount) the wrack? Did the remains of La Gloriosa continue on from there through the "seas-became-strange" portal into the seas of another world, leaving pursuers behind?
Would any survivors who were blown clear be able to discern where the rest of the ship went after that? If the ship sank as they watched, could they find the spot again? If it was near land, there might have been visible landmarks; but then, why did they take this long to tell people what those landmarks were? (Temporary or magical amnesia?)
I think it's likely Quickleaf already has answers to most of those questions, and is saving them for the party to discover later. For now, I'll hold off on trying to overwrite that part of the setting with my character's attempted backstory; but those are really intriguing ideas all the same.
2) I wouldn't worry much about the Resistance cantrip; I'm fairly sure the curse that overtook the Coral Curse was stronger than that. But it is worth wondering why he was spared that fate. Perhaps he was NOT...and he only is holding at bay for now? The cure may be in the Gloriosa. Or perhaps he appealed to some loa for protection, and that protection was granted in exchange for some item or service related to the Gloriosa?
So he
survived the epidemic but never fully
recovered from it? That's a compelling concept in its own right.
Umm. I think that not deciding finally on a Bond is a good idea at the moment, with this proviso: for now, before the character gets built to match, he has three Bonds to La Gloriosa! They are: Vengeance/Justice, either to find the guilty french privateers who aided in the raid on Cartagena, or to clear his own family of suspicion and accusations of having been among those; Redemption because he failed to find La Gloriosa before; and Recovery, because the Yellow Fever isn't gone from his body, it's merely in abeyance.
Bonus stuff:
(0) Was Cartagena named after the historical Carthage? Might we expect Hannibal and elephants? 
(1) The reason nobody can find La Gloriosa del Mar by scrying magic is that, while they were under pursuit, the people aboard her held a ceremony to re-christen the vessel, using a mixture of French and Spanish to throw off investigators, renaming it "La Furiosa del Gloire." Nobody would think to scry for its location under that name!
(2) I finally did watch Swiss Family Robinson on Sunday. There were pirates in the movie, but they were more the plot device and the villains than being main characters. That's to be expected for that story.
More connections: SWR featured James MacArthur as Fritz, the eldest son, and John Mills as the father, with Janet Munro as the girl who is first mistaken for a boy.
(a) What, Munro died at the tender age of 38, in 1971? (I had seen her before in Darby O'Gill and the Little People.)
(b) John Mills and James MacArthur teamed up again in "The Truth About Spring," featuring a maturing Hayley Mills as the girl (tomboy in her case) who is first mistaken for a boy.