We have vastly different experiences with parents, so I can in many ways sympathize with your own biases. I may not have put it well but the abusive and traumatic methods of attempting to "correct" gender dysphoria should of course be heavily punished, same as for any other abuse. That includes loss of parental rights in cases that justify such action such as extreme neglect and abuse both mental and physical. Remember though foster care isn't an ideal solution either, as people who volunteer as foster parents, at least in the States, tend to do it for either finical reasons, or for faith based reasons. But that's also somewhat beside the point.
Now if it comes to mental abuse for being trans and a child's therapist gets wind of it, they had better damn well report it to child protective services, same goes for physical abuse and neglect. But this isn't always the case. Also for treatment the parents deserve a say if they're not actively trying to abuse their child, due in no small part as a means of attempting to protect the child's welfare. Since bullies will relentlessly target trans students, at least in my experience. Another consideration is who's paying for any treatment for their child, if it's the parents, then they have a say on a purely financial basis. Since treatments for trans are often seen as elective, most insurance, as far as I recall, doesn't have to cover it, but the Affordable Care Act might have changed that. I don't remember off hand. Either way the parents still have a financial stand there too as they're the ones paying for the policy and deductible.
If you're living in your parent's home, then it's not unreasonable for them to expect you to live by their rules, unless it's active abuse. You could argue that denial of treatment for gender dysphoria is abuse, but on the other hand it could be argued otherwise, especially on a case by case basis. Some kids may be fine not transitioning yet, many though hide the condition actively just to fit in.
In the US teens can emancipate themselves from their parents somewhere around age 16 in most states, so if they're that age, they can totally cut themselves and their medical records off from their parents. Which sadly might be the only option for quite a few trans people. Though one thing I do remember is that in most states you have to be 18 years of age to get a fully unrestricted driver's license. Even so I don't trust anyone under 21 years old behind the wheel of a car, younger people(hate to make my self sound old) are maniacs behind the wheel, and usually lack experience. Also in the US you have to be 18 years old minimum to consent to contract, without parental consent also. Though emancipated teens may get a exemption on that rule.
Anyways, as much faith as I may have in kids and I know they're not stupid, it's still a bit of a reach to expect them to make important medical decisions. We are talking about an age group that's well known for inflicting injury on themselves by making really dumb decisions. But hey, that's part of growing up. I'll trust them more as they get more information and experience under their belts. Still how many will out right defy even their own parents, when the parents give them some very commonsense advice.
Parent's don't own their children, but until they're grown up enough they are responsible for their well being and upbringing. So parents need control over their kids, not just to command them, but to make sure they're safe, healthy, clothed, fed, and generally taken care if. That often trades off to the kids not having full freedom, which honestly is a good things, too many kids end up in danger, dead, or in jail when not looked after.