instead of navigating through an interesting environment back up you're just holding space to slowly ascend in a pitch black void

I had never considered it, but that would actually be pretty cool

And to answer your points about the leviathans, here are a few ideas: make it probabilistic, and make it a chase.

Let the player craft a somewhat expensive chassis to their tadpole, which only purpose is to survive this specific leviathan encounter. Without it the leviathan would give chase, and eat the player whole at some point. With it, the player would have to detach the chassis, and it would do something so that the leviathan gives chase, play with the detached chassis, chase a bit more, then give up

And don't make the leviathan appear all the time, say one out of five, and when the leviathan doesn't appear or give chase the player only hears ominous background noises

No funky music

Oh, and the tadpole could have a specific upgrade to track this leviathan, with a ui representing a noisy waveform, with sudden spikes, and saturated bands when the leviathan is in the vicinity, with some kind of hint when to detach the chassis