While I agree with you that a good portion of Hanahaki disease fic is amatonormative, it's actually possible to do with nonromantic feelings or even in the sense of "I don't return your feelings and I cannot do so...but I am here for you and you do not have to hide and suffocate under the weight. I am not IN love with you but I do love you because you are very dear to me".
In fact, according to this post (Source: https://www.tumblr.com/storm-does-stuff/803773290007183360?source=share), the original intention is less "My feelings must be reciprocated or I shall die" and more "Repression never ends well" . People just took "The cure is confession" and ran with the inherently romantic "The feelings being confessed means they must be returned" instead of using "I am glad you feel comfortable sharing such delicate feelings with me, even if I do not share the romantic inclination" or other less romantic-coded outcomes. And this even allows for angst of "I can't believe they weren't comfortable enough with me to feel they could tell me about this! What have I done?!" without the potentially distasteful "I killed them/made them sick because I didn't reciprocate the feelings".
Still do what you love but it's a case of "It's not necessarily amatonormative; just spoiled by people who hold those beliefs".
As gently as possible, this still doesn't really address a lot of what I find horrifying about Hanahaki as an idea. Frankly, whether it's romantic or platonic (and I have a whole other slew of feelings about the idea that one can make a trope aro-inclusive just by making things platonic instead, because society just doesn't treat those two things the same), the expected reciprocity is the real problem. No one should be forced to feel a particular way about someone else, period, and as a trope it sort of inherently robs one person of the ability to say "I am not responsible for how you feel about my response"
I guess if I were to frame it a particular way: I've been friends with several people over the years who really made me feel personally responsible for their well-being. I was told my friendship and generosity towards them was critical to their mental health, and it was not-so-subtly implied that the people in question might kill themselves if I ever retracted it. As a result, I felt very trapped in those unhealthy relationships, even when they really wore me. And that was wrong of them. People do have a responsibility to support others to a certain extent, but no one ever has to sacrifice their own mental health or well-being for someone else's. In the slightly-tweaked immortal words of tumblr's favorite skeleton: If it sucks, you can in fact hit da bricks.
Like, the problem with the trope isn't really even the romance (though it has sort of metastasized there in a particular way), it's the coerciveness. And there just really isn't any way to make "I feel a certain way about you and it's making me sick" not coercive. Which is to say: if other people want to toy around with other takes on it, that's their right, but I'm never going to like it and can only see it as a horror trope, personally.
(Also, I'm not sure what that other tumblr user's sources are, but a quick dip into the trope's Fanlore page and some further internet searches suggests that the manga that gave the idea its name/codified what fans do with it seems to have played it pretty straight, so I'm skeptical that the "original intent" isn't supposed to be wildly amatonormative in the first place.)
