First let's start with>Central conflict on the island
and create some factions. You can't have ALL lizardmen ALL the time unless you heavily rely on flora and fauna.
Clearly the island is archaeologically important, AND there's weird shit going on (mechanical bull), AND it was abandoned for good reason. Maybe there was a war.
So FACTION 2. The mechanical bull faction. The obvious answer is "gnomes", but if you hate gnomes or want something a little different... warforged. Built by an ancient highly advanced Mayainca empire. Maybe giants. Maybe some of these giants survive as brains-in-jars or magical cyborgs. Maybe they're sleeping or long dead. In all cases the warforged carry on their last will. Take plenty of inspiration from Norse mythology and weird Maya astronaut theory bullshit.
FACTION 3. Does your island have a volcano? It does now. Anyway the place is teeming with either devils or fire elementals/primordials. They're the lizardmen's manipulative new gods.
Which brings us back to FACTION 1, the lizardmen. Why do they hate the PCs? Being xenophobic and territorial is enough but then they'll always be reactive, not proactive. It's better if they send hunting parties to capture the PCs, and generally want something from the PCs. Maybe they think the PCs have got magical blood coursing through their veins so they're trying to sacrifice them to their dead gods to bring them back. Maybe their new gods want to make the PCs their vessels or something.
So what happened on this island? The giants came into conflict with a 2nd precursor race (maybe yuan-ti or mindflayers). Maybe the lizardmen were once their thralls, maybe they consider them "the old gods". Either way, the 2nd precursor race hid deep underground and the giants couldn't fight them, so they created smaller warforged soldiers. Then something happened. One side made a magical nuke, drew on fiendish/primordial power, and wiped out both races. The volcano has been active ever since.