Have you run tests comparing your game to the Braille edition of Scrabble? If you haven't already, you might seek to specifically recruit current players of Braille Scrabble. That may still only represent a fraction of the game players you'd like to reach, but experienced players may give you a lot of good feedback quickly.
For browser/device info, check out the WebAIM surveys. There's a lot of data to mull over:
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