Of course, the TDEE calculator is just a starting point. And yes, illnesses add complication, but you're making it out like it's impossible. There's some number between 0 and your current intake that will just sustain your current weight. Between trial an error, calculations, and a doctor, it can be figured out. Eating 100 less than your TDEE is of course not going to produce much of an effect (that's like, one less chocolate truffle a day) , you need to go lower. Yes, 'extreme diets' are not usually sustainable but the point here is to figure out a number, not stay on that diet forever.

Look, sometimes I feel like I'm attacking fat people, but there's a reason I'm so wary of excuses. The brain really wants to eat more food. In our evolutionary past, this was a beneficial thing, and all the checks and balances on it came from the actual scarcity of food and the work required to get it. In modern society, it takes active will power to fight the temptation. So our brains invent all kinds of excuses that let us eat more and get that dopamine rush. We make decisions based on our sub-conscious desire for food, then invent some narrative on top of it about how its a cheat day, well it's whole-wheat, I don't bother since my thyroid is sick, etc. I see it all the time with well-intentioned people who don't even realize they're full of shit. I had a friend who started losing weight around the same time I did. We were both healthy young people who were marginally overweight. I worked hard for months and deprived myself of the pleasure of shitty food (which I craved every waking second). He kept complaining about not losing any weight, saying he wasn't doing anything egregiously bad like gorging on McDonald's, but every meal seamed to involve an extra side dish (it's small!), every weekend seemed to be a cheat day, every high-calorie meal was okay because he ran a mile yesterday. Things that would have made me seem too harsh to point out individually, but that added up over time. Because his brain wanted X calories and it constructed whatever narratives were necessary for that amount of food to make it into his mouth. I'm not trying to go after fat people, I actually also see them as victims of a deceitful will deep within their brains. I just like to help them fight that will and realize that what their brain is telling them is often bullshit.