Well the works already been done for you with the super modules A1-4 Scourge of the Slave Lords and GDQ 1-7 Queen of the Spiders. The only difference is substituting the Saltmarsh adventures for T1-4 The Temple of Elemental Evil that served as the intro for the other two.
In U1 the PCs can get a pirate ship and sail it around a bit to take on the lizards and the sahuagin in U2 and U3. So presuming they're successful and feeling their oats a bit, maybe they feel like they're good enough to take a trip up the Wild Coast in the old Sea Ghost. Once in Safeton they get an invite to Madame Gold's gala feast (which just happens to coincide with a slaver raid) and you've got the start to A1-4 all set, with the added bonus that the PCs have a pirate ship allowing for a lot of cool possibilities on their way to Highport.
If you don't have the supermodules, the key to tying them altogether is that the drow Slave Lord (Edralve, I think) is working for the Lolth houses of Erelhei-Cinlu and Stalmin Klim is in bed with the Elder Elemental Eye houses under Eclavdra (or maybe vice versa), so that both sides in the GDQ series have their fingers in the pot that is upset by the PCs and have incentive to want to off them when they get involved in the giant raids.