@Hip, I sympathise! Reading's hard for me too. I have autism with ADD (at least I missed out on the H!) with ME on top, and if I work really hard at it, I can make myself read for 10 minutes That's why I call in on PR so often; at any one time I've got a couple of books, at least two TV programmes, a social media page, and maybe a radio documentary on the go, but I need to keep switching between them or my brain hurts!
To be honest, the only way to optimise typefaces for any group of people is to make it possible to change them on web sites, along with backgrounds, levels of contrast, and colour. I've no idea how to do this, but I think you can do so in a basic way on some message boards. I get so distracted that I even have a blank browser window covering the sidebar on PR – changes of colour and photos just draw my eyes away all the time, and it's exhausting. If I've got a long article to read in a difficult typeface, I'll often paste it into a Word doc to make it readable and remove distractions.
The reason most people use Arial/Helvetica on web sites and correspondence is that it's the best available compromise for the broadest range of people – but that doesn't mean it suits everyone. There's a specially designed typeface called Dyslexie that I used to use when I worked on arts projects with groups of dyslexic kids; I could never compose text in it, though, because it's a ruddy nightmare to read if you don't happen to be dyslexic!