I would still do first name, but "Dear First Name"

With a small number of exceptions (e.g. service animals), most accommodations could be extended to the entire population without any serious issues. You can design tests to accurately evaluate student understanding in such a way that extra time is not an unfair advantage.

We already face nonsense where everyone who isn't a white man is accused of being a "DEI Hire", and we certainly don't need the same pernicious approach accusing disabled people in leadership positions of being "Accommodations hires"

As someone who has worked in academia for over a decade, this is not a real problem. It is incredibly challenging for disabled people to progress to anywhere near the top ranks of our education system.

Universal Design solves a lot of these problems, as well as recognizing that the vast majority of the time there is little harm by extending accessibility measures to everyone who wants them. Let people who work best remote work remote, don't design tests in school to be a speed run, etc.