What I find most evocative in this setting is your description of the possible dungeons within the Titan's corpse. With the spear having turned some things to nature, lava getting in, water getting in, etc. you're giving DMs a ton of creative space to work with! In my mind I keep thinking back to Hyper Light Drifter, a lot of visuals from that game affect how I visualize these dungeons.

You mentioned that adventurers might find magic items here. And now it's looking like they'll encounter fiends in various stages of imprisonment, I love it. My question is, why would there be usable magic items inside the Titan? Of course there would be magical mechanisms and power sources, but what makes them usable by PCs?

I suggest that people would have been stationed inside the Titan as "crew," and were part of its operation. Now if they directly controlled the Titan that'd just be cheesy, like Power Rangers, so I wouldn't go that far. Perhaps their tasks would be maintenance related. Maybe if you left it open just what kinds of tasks the "crew" took on, you could give DMs creative space to have "Mind Mines," prisons (I'm thinking of the Death Star,) research complexes, etc. And naturally you'd have a plausible explanation for usable magic items lying around.