I agree with all of that, with an added bold-faced addition about others' perspectives: ensure that your research centers around what people in that situation have written & said about their lives. That especially applies if the character has a disability, as "experts" and parents can only describe their conclusions based on what they see from the outside or what we tell them, and can't actually give you the internal viewpoint one of us has.
To get back to the point of the article/letters, I was passionate enough when writing about being autistic firsthand to read everything I could find by others on the spectrum. Their perspectives significantly improved the quality of my work, based on the feedback I was getting, as they had quite different ways of describing (and sometimes percieving) things, which let me see some of my own assumptions about the topic.
One favorite I recall, which I've used to help my writing in general, was the idea of distinguishing between how a person/we sees their/our traits, and where they/we have internalized others' opinions. Simple concept, but it's interesting and useful to go over one's characters (or self-perceptions) that way and see what pops up. (For example, offhand: does Suse hate her lingering accent on her own, or perhaps that boy that's overly friendly in chapter 6 was vicious about it when they were little, until...)
Research what you care about -- before writing it!
One of the interesting writing blogs I read, Writerly Life, has a routine feature called the Mailbag, where the author quotes some of the responses that were left in an article's comments section. I missed the original piece Write About What You Care About, but I did catch the follow-up for it. Most of the advice people referred to (or offered) sounded spot-on right, but there was one little issue that left me uncomfortable, so I felt compelled to offer a response...