I think a good approach could be to think about how you could reach users of different platforms.
A lot of Mastodon users follow hashtags, so including relevant hashtags (#accessibility and #blind seem like good starting points) might be a good idea. Tagging groups, such as @accessibility, might also help.
I think Kbin/Mbin might be better suited for this than Lemmy, as it integrates better with other federated networks. You can follow microbloggers and boost content, which in turn makes them likely to follow you back and creates a community beyond which Lemmy community you choose to post in. Your Mastodon followers will see your posts, but it won't matter to them which community you post it in.
It's hard for content to make the jump from Lemmy to Mastodon as Lemmy does not make itself discoverable, but as soon as content reaches Mastodon users nothing stops them from interacting with it (by boosting or replying).
Sadly Kbin.social lacks sufficiently active moderation these days, so you might be better off with an mbin instance. I also have no idea how accessible Mbin is to blind users.
Edit: I over-emphasized the point about reaching a broader audience. If you want to discuss a narrow topic but you don't want most ActivityPub users to see it because you don't value their input, I guess Lemmy is as good as it gets.