Thoughts on truncating this adventure and just basing out of Sasserine?
Go to the isle, maybe help Farshore but change the plot?
Shift the focus to Farshore or Sasserine?
You could do it. It'd need some rewriting though. Regardless of the solution you chose, the main issues would be around the Tides of Dread adventure. It's kind of a colony-bulding effort, bit of a quasi-hexcrawl, and culminates with an invasion of Farshore - a bunch of pirates, some demons, Vanthus etc. It's fairly small scale, the PCs can coordinate the defense and be the big heroes. If the attack is instead on Sasserine, then it by necessity becomes a much, much larger-scale thing, major wizards will get involved, all of Sasserine will be up in arms etc. Even to mount an assault on that scale, Vanthus would need to basically mobilise all of Scuttlecove, and the campaign isn't remotely ready for THAT yet. The PCs are (from memory) not even 10th level at this stage - you'd need to make sure the focus remains on them. Why hasn't anyone done any hexcrawling, or hunted the Great Beasts, or sought out Zotzilaha, before they got here? And justify a reason why the PCs are the ones who have to troop into the underdark after the aboleths, or head to the City of Broken Idols. If they're the Big Damn Heroes of Farshore it makes sense that everyone turns to them, if they're just another set of adventurers in much-larger-Sasserine,then maybe less so.
Option 1 could be to replace Farshore with Sasserine as the city on the Isle Of Dread. Start the campaign in media res, with the PCs and Lavinia on a ship heading towards the Isle of Dread. You'd need to replace the first couple of land-bound adventures, and in the process find alternative ways to introduce the Vanderborens, the concept of the savage tide, etc etc. Other problem with this approach is the same as the campaign as written - after Tides of Dread the PCs won't actually spend much time in their home port of Sasserine, other than downtime. There's two adventures in the inland of the Isle, then one in Scuttlecove, then the rest of the campaign is mostly extraplanar.
Option 2 makes Sasserine the centre of the whole thing, and the City Of Broken Idols is actually just a bit inland of Sasserine itself, in which case the Sea Wyvern's Wake never happens, and Here There Be Monsters never happens (or perhaps Sea Wyvern's wake a trip out to ... somewhere ... to pursue Vanthus after Bullywug Gambit, and then the ship gets wrecked and Here There Be Monsters is the PCs trying to return to Sasserine). Again, you run into the whole issue of 'this jungle is stuffed full of demons etc and there's a whole big city over there and WHY HAS NOBODY DONE ANYTHING ABOUT IT BEFORE NOW??'
Or perhaps Farshore is a relatively small but secret Vanderboren commercial venture (perhaps a gem mine, or maybe cultivating rare herbs or something) just a week or so by ship down the coast from Sasserine, which would make travel between the two much easier. Which means at least the PCs can meaningfully defend it against Vanthus in Tides of Dread without all of Sasserine mobilising, but could still pop back to Sasserine without too much difficulty afterwards.
If you're looking to truncate, the most obvious point is after City Of Broken Idols. The Aspect of Demogorgon there is the big bad of the whole thing, no need to go to the abyss, end of story. Alternatively, you could also stop after Tides of Dread - Vanthus's attack on Farshore is defeated, the PCs beat him down against the dual timebombs of his Shadow Pearl and the vrocks performing a Dance of Ruin. The loose plot thread there is of course where the Shadow Pearls come from in the first place, but as a final mini-dungeon crawl give Vanthus a giant demonic flagship encrusted with monstrously deformed fiendish oysters and that'll pretty much do the job...