@NightFury oh wow, that sounds overwhelming, to say the least! Must have changed a bit since I got diagnosed in 2018... I did have to talk about how my 'symptoms' (I just hate calling them that. I prefer 'traits'. But I digress...) had been present since childhood, and that my Mum displays similar traits, but that was about it. Is there a possibility of chatting to them about whether you need your parents to fill it out, or whether there's another option? Cos like... not everyone speaks to/has parents, so there has to be backup options for when that isn't a possibility - surely!
I agree with @MissGremlin that it might be helpful to chat to one of your supports about a 'game plan' or something. It could help to have some resources to share with them - which, please hit me up for if you need some, cos I have oodles haha.
Also, I've had to 'come out' to my parents a few times (first as gay, then ADHD, then nonbinary... lol) and something I've learned is that their first reaction is NOT their 'full' or their most 'accurate' reaction. My parents went into a bit of shock each time, and did say some... not great things to me. After a couple of days once they'd had some time to process (and look through the resources I showed em) they were more able to be open and curious. Your mileage may vary of course, depending on your parents' beliefs and how they typically react to stuff.
Buuut...we typically get our ADHD from one or both of our parents so it's always a little interesting to see how they respond. I think for my mum (who I get mine from) her shock started to wane a lot more once I started pointing out all the traits to her and she just kept going 'oh that's like me!'
In any case, remember that this is your journey, and no one else gets to decide the pace or path you take 💜