I am a 48 year old graduate student who was diagnosed with ADHD a few years ago. I first of all want to applaud you for your courage in asking for accommodations as you complete your PhD program. When I decided to ask for what I needed as I embarked but the master degree, it felt like such a big step. I’m grateful to hear of someone at the PhD level and as a recently diagnosed student ask for what you need. That said the ADHD coaching group sounds like a good resource. What I can tell you is what I learned from a very helpful disabilities support counselor. If an assignment is considered a final exam but is not given in a classroom setting where a typical extra 50% time could be added to the time the exam is administered, students may successfully argue since the the paper is in essence, a final exam the same extensions afforded during classroom exams should be offered in order to provide equity to you as a student with a learning difference. Below is a link to extended time guidance from JHU that might contain some useful language that your support the idea that in keeping with your accommodations as suggested by a doctor, you need the extra 50% time. So if everyone else received 4 days, perhaps you request an additional 2 day.
Best of luck!