Hi, I'm the founder of Mailmeteor, the best-rated mail merge for Gmail.
I did accessibility tests in the past and had classes with students whom I taught how to use mail merge with Gmail. Two of the students are blind and could use the tool without a problem.
I'll be glad if you let me know how any accessibility issues that you may encounter and also to help you use the tool.
See the Mailmeteor's Google listing here:
$1Website: https://mailmeteor.com