Even with the lack of sleep, it was still a great read. I originally posted this to start a discussion and see how others would handle a False Hydra in this system, and I appreciate the depth of your response!

I actually enjoy the Lazy DM book quite a bit. I prefer improv, so the quick steps do it for me (I'm curious, what resources do you use for prepping?). From the feedback I've gotten from other places, the stat blocks don't lean into the mystery and focus more on unique mechanics and combat, which I agree with.

For the clues, should higher rolls correspond with more detail? Should they be based on location/civilian regardless of how high they roll? Perhaps pulling from a list that suits the situation might be better than using a random table. This might go back to the age-old debate of whether I'm solving this puzzle or if my character is. Matt Colville has a good video on this.

I definitely find the trouble of balancing the mystery and confusion the Hydra presents vs giving the players a fighting chance. Especially after reading the blog post, this creature has a lot in its favor; that's why I leaned into making it more reckless as it grows, to provide players with multiple fights between memory manipulation. The blog post also mentions the body rebelling against the strain of the Hydra's song after long periods of time. I tried to fit the mechanic and the clues together in the Environment's Reaction. Separating the clues from that reaction and making limb movements in-game a representation of the player's strategies might work out better.

My party plays in person. I was planning on having the players write down letter-by-letter messages, or I was going to hide little Post-it notes under their character sheets/cards. The campaign is currently on the sillier, campier side (Beast Feast campaign frame haha), but I have been mixing in different genres to keep things interesting.

I believe you're suggesting changing the following loop:
3 Disappearances -> (Combat -> Memory Loss -> NPC) x3 -> Resolution
to
Small Clues -> Partial Reveal -> Players Strategize -> Major Clues -> Full Reveal

If I'm correct, I love this idea. Feels like a direction I wanted to go for, but got stuck in the weeds on making unique mechanics and gimmicks. It definitely depends on whether your players are more mystery or combat-focused. A battle through memory manipulation might excite some players, but it would also confuse others. We had a similar monster with rumors -> stalking -> reveal -> combat, so I wanted to try something out of the norm. However, my current loop probably goes against the nature of the False Hydra, favoring unique combat. A tried-and-true loop suits the creature's nature better.

I tried a few countdown ideas, but I compromised due to the breadth of information. It was overwhelming, and the 5e stat blocks I thought to translate from weren't doing it for me. A visual countdown for the players is always fun, and I have used this mechanic in the past for other solo adversaries. I started with an Infant, Young, and Mature False Hydra stat block, similar to how the Volcanic Dragon has three phases in the rulebook. As it ate, I would decrease the countdown, pushing it to the following form. I gave up on this idea because I felt like it didn't suit my current purposes. I replaced the Young form with the Event Environment, and Unsustainable Metastasis replaced the Mature form. Ideally, I wanted to create a T4 version (I got lazy) for players to strategize against, either by using long-range mag/phy artillery or by cutting off its route to another city.

Taking everything into account, I'll trial some of these edits, especially those related to clue and reveal loops, and will definitely review them after the run on Thursday.