Regarding the season one finale, that was indeed the core that they sort of slingshotted around/were pulled through. The tunnels – the snake pits – seem to form some sort of a transportation system linking parts of the planet through/around the core.
Regarding season two, there's definitely some wonky CGI (especially in the first few episodes), though I found that I really loved all the design work so even when the effects weren't entirely up to snuff, the actual visual language of what was on screen was doing the work and getting me invested in what I was seeing. I love the whole look overall of the Tropical Zone. And there are some pretty big special effects sequences in the last few episodes that look phenomenal, in my opinion – it feels like maybe they shaved a few dollars off the first few episodes' effects budget in order to get that stuff near the end really looking top notch.