Good work overall on the pacing and on the themes, it is pretty easy to figure out where to go.
Though I do wish the thok barriers could be lowered a bit (that is unless the ravine feel is intentional). The main areas that need attention is the last two acts of Lava Core. The second act places you at a risk of dying right away in your start position, and in the third act, the diagonal walk cycles of Brak mean it can be difficult to get his shield down.
Also, congrats on finding a use for the music of Sonic Spinball's lava table, perhaps the first time I heard it while playing myself in nearly 20 years.
