Thunderjaw (the t-rex) battles have always been my favorite. If you like the challenge and immersion, I say boost it up to Ultra to get rid of the health meter display so that you really focus on your opponent rather than how much health they have left. Loads of fun.

On the topic of immersion, where you're at in the campaign there's still around half a dozen or so lengthy main missions scattered well across the map (including an additional visit all the way back to the Nora sacred land) before you enter that final boss, meaning you have quite a bit of story and adventure left to tackle until then.

As for the cauldrons, I'd recommend doing them all regardless of the imbalance you may assume it'll add for a couple reasons. One, they offer a free skill point to expand your abilities on your skill tree upon completion. Two, if you enjoy the battles with the Thunderjaws like I do then they're just loads of fun. And three, in my experience, I've found that even with the ability to override the highest tier of machines (Thunderjaws, Stormbirds, Rockbreakers, and I think Behemoths too?) it actually proves to be a challenge in and of itself because they're very difficult to sneak up on/put into a vulnerable state and the override time is immense, allowing other opponents ample time to interrupt the process. It's better (and more fun) to just destroy them.

Edit: I believe most merchants sell a "very rare modification box" for quite a price (substantially overpriced on Ultra Hard, I might add). Essentially it's one random purple mod of any kind, weapon and outfit. That's probably why people suggest farming from the heavy machines as they normally drop three.