Excellent
My idea is (and was long before I knew how ToA started) to feature a portal in Red Larch (the Sword Coast) than deposits the party right into a forgotten tomb in Chult.
Assuming Hidden Shrine of Tamachoan works out, the idea is to put in close to Fort Belurian. The Flaming Fist is portrayed as a bit too ineffectual for my taste, having had essentially zero success in finding and looting tombs themselves.
So the uppermost layer/level of Tamachoan (or whatever) has been explored by the Baldur's Gate mercenaries. Then the party emerges from hidden depths - thereby revealing how to get to the lower levels. As soon as they try to escape the tomb they will be confronted by a squad of Flaming Fist guards posted at the supposedly emptied site. (Assuming they don't prank them by posing as tomb guardians, and have to kill them all, that is
)
This accomplishes several things: they start
in media res, always good. (They're about to level up to 5th level which seems to match Tamachoan well). They
immediately get a dose of what the module is about: exploration and tomb raiding
They run afoul of Flaming Fist directly, and their insistence on confiscating a half share will be a real issue. (Otherwise, I fear you can just kill off random patrols and make sure you're not caught, and pretty much ignore them, tucked away in a corner far away from the real action as they are). This way, there is riches right under their noses, and the party will have to decide if its worth risking their lives exploring the Shrine essentially doing the Flaming Fist's job for them, or if they should abandon it to move to Port Nyanzaru instead).
I quite like the idea to first reach a smaller town, so the rumors about PN can build up instead of 'porting right in. At Fort Belurian they will have everything they need to get up to speed: info on their whereabouts, supplies, rumors (including that of the Soulmonger), and even a few guides.
As for "Syndra's map" I think I'll just handwave it. Instead of making it a highly coveted physical in-game map, I'll just give it out as part of the impending hexcrawl.