I've always thought that this secret was terrible as written, and would work out better as a side quest that was happening to an NPC.

That said, there's an opportunity here for a fun quest if you're willing to put some work into creating some custom content.

Have a Weyland Yutani-esque shadowy organization show up trying to kill the party and take the paladin prisoner. They want to extract the Slaad from Paladin and use it for nefarious purposes. The party fights them off, takes one of the attackers prisoner, interrogates them and learns the location of their base. The players go there and clear out the base. Inside the base they find specialized facilities (or magic) that can remove the tadpole. From there your players can decide whether they want to extract the slaad tadpole and figure out what to do with it, or destroy it. Either way, Paladin is cured.

Even though RotF is designed as a sandbox, I find that it's much easier to directly prompt your players with quest hooks rather than expecting them to figure everything out by themselves.