Here, bunch of stuff to consider (from mostly an outsider perspective though):
Posting to the group is a good idea, I’d continue doing that. But also consider looking for relevant hashtags popular among the people you are trying to reach, and always include the hashtags in your posts. I think it's much more common to follow hashtags than groups, meaning that that’s how you’ll probably reach the largest number of interested people. Posting to groups may help your posts to get distributed wider in the fediverse, boosting the effect of hashtags more.
I think a huge problem with Mastodon is that people who have busy feeds very often miss posts by their close peers if the latter aren’t very active or if their online times overlap only rarely (eg. due to time zones). This can easily lead to isolation and feeling ignored, which I believe silently hurts Mastodon’s communities. If you have a busy feed, you may want to use the list feature and maintain a list of people whose rare posts you don’t want to accidentally miss. Eg. people who you want to help feel less isolated and more welcome, or just people with whom you have had meaningful interactions before. However, because you can’t of course insert yourself to their lists, this doesn’t help you increase the probability of getting replies to your own posts. At least not directly.
Explicit calls for help and boosts probably work well, just like you did now. But try putting them to the beginning of the post or CW header, and/or maybe add an emoji to better draw attention to it. Include a short explanation about why you need help, it should motivate better, eg: “Please help/boost! I need to connect to other neurodivergent people, but I rarely get any responses and feel isolated. Has anyone seen my post?” Or add something like it plus a tl;dr in a reply later, if the original post went unnoticed. Don’t forget hashtags in the reply.
Also explore different times of day and weekdays. When most of the people you want to connect with seem to be online? When they are most sociable? Could there sometimes be too many people online and the hashtags/group feeds are flooded? Can the quietest hours actually be better?
Ask people who they think might have an opinion about the matter you want to talk about, or who might be able to help in some way, or might have much in common with you. Many people may have much lower threshold to ping someone they know than write what they themselves think. And people who are explicitly asked for their help are probably more motivated to help, compared to the case where they only see your post in their feed.
Finally, people have much lower threshold to answer a poll than to like or boost or reply to any kind of post. If you can, make a poll.