Just past the lava zone, there are three tiles of obsidian at the bottom, each with a toughness of 30. Even with max Power, they'll take around 20-ish stamina to break through. Once you dig past those, you'll reach the current soft ending of the game for now.
As for turtles, so far there are just the default (Scoot) and Grampshell, who was recently added. But more turtles (and different ways to unlock them) will be added from time to time.
The Jackpot on the slot machine is very rare, about a 1 in 593 chance on average. I'd recommend holding off on grinding too hard for it just yet... I'm working on better rewards and special unlockables tied to the Jackpot :)