Hey, good questions. Style-wise think low poly / stylized, not photoreal. As long as it reads immediately as "volcanic island," we're good. Think beach, cliffs, active volcano with some particle FX on it, nothing crazy. I'll drop a couple refs below so we're on the same page.
Budget's $90–140, timeline's 5-7 days but I'm not in a rush, so don't stress the clock. Let me know if that's in your range.
Best reference: Sea of Thieves — The Devil's Roar Active volcanic islands, smoke, lava, tremors. Stylized but not cartoony. This is basically the vibe. https://www.google.com/search?q=Sea+of+Thieves+Devils+Roar+volcanic+islands&tbm=isch
Also good: Wind Waker — Dragon Roost Island Shows how beach/rock/volcano blend together on one island without looking messy. https://www.google.com/search?q=Wind+Waker+Dragon+Roost+Island&tbm=isch
For the real-world shape: Hawaii aerial shots https://www.google.com/search?q=hawaii+volcano+island+aerial+view&tbm=isch
For the game-art look: low poly volcanic island search https://www.google.com/search?q=low+poly+volcanic+island+game+environment&tbm=isch