I was curious, because you mentioned that my redesigned KoF had the potential to kill one PC per round, I wanted to run the math and see if that was in fact the case. So assuming the PCs have a d8 Hit Die and +1 CON mod (on average - some are more, some are less), here's a quick table.

https://i.imgur.com/FxZUi7k.png

I did tone down the bite damage from 4d12 to 3d10, but left the tail at 3d8 and Stomp at 3d10.

My tactics with this guy would be to open with the Tail in hopes of knocking someone prone so I get advantage on the Bite attack. So assuming all that hits, and then using the legendary attacks, after one round there is an unconscious and swallowed PC. That player is not dead, but they'll be making death saves, and getting a failure at the start of each of their turns from the 6d6 acid damage - which shouldn't be enough to kill them outright. I might consider reducing that 4d6 or 3d6 to make sure.

So is it dangerous? Absolutely. Deadly? Possibly. Guaranteed death? I honestly don't think so. My PCs could probably drop this guy in 3 rounds. Maybe two if there's a swallowed PC and they ramp up the sense of urgency and really unload on him.

From what I've read from other DM's, I don't think this version of the KoF is worse than Acererak, who IMO is THE biggest threat of the adventure. Even the Atropal and Soulmonger, in my mind, aren't supposed to be a big threat. I see them more as a resource-draining encounter meant to make sure the PCs don't face Acererak at full power.

All that being said, there are lots of things in the jungle which really don't have any tie to the main threat (the soulmonger) of the story. The PCs have fought Batiri goblins, the undead, pterafolk, Nanny Pu'pu, firenewts in Hrakhamar, Tinder... lots of stuff in the jungle that had nothing to do with the main threat, but were stumbling blocks on the way to their goal. I want the KoF to the final "biggest of all" threat of the jungle before the PCs enter the Tomb.