The idea is solid, the implementation may leave something too be desired. A hesitation I have is that Lemmy is new and unstable. It can be tricky getting things to work right for the average person, I can't imagine it's an easy time for anyone with higher accessibility requirements.
Unable to suitably categorise Posts (sub-group/topic type/region)
I think the only thing on Lemmy that helps with this is creating different communities on an instance. There is currently no tagging of posts or anything like that.
Difficulty searching/filtering
I'm not sure the search on lemmy is all that great? And with no tagging, filtering is pretty non-existent.
Posts ‘drop off’ the bottom (due to inability to filter)
Lemmy lets you go through each page so you shouldn't have a problem with not being able to go back far enough. It also allows you to sort by new comments so it's easy to find the things people are commenting on.
Continual hacking risk
This is a risk for any platform. I guess one benefit is you have your own backups, so if something happens you can restore from a backup. But I'd think Lemmy is much more likely to get hacked than a facebook group (with admins using 2FA). It wasn't that long ago that a security issue in the Lemmy software caused the largest Lemmy instance to have an admin account taken over by a malicious user.
Platform controlled by off-shore corporate interests
This is a benefit of Lemmy. You control it, you host it, you own it. You can choose which other servers you federate with (if any).
Also, it seems like this might be something that is helpful to get a wider range of opinions on. The community on !newzealand@lemmy.nz is significantly larger than this !support community, perhaps you could cross post it there as well? (there is a little icon under the post for cross posting if you're accessing Lemmy from a browser).
Also r/blind set up a Lemmy server at https://rblind.com and they may have some suggestions on whether it's suitable.
One thing that may help that people often don't know, when you post a picture in Lemmy (or anywhere using markdown), you can put alt text so blind users can have their screen reader tell them what's in the image. You put the alt text in the [], like so:

Hey Dave, I really appreciate your in-depth response.
Good to hear honest feelings about lemmy and some of its problems.
If I were to use lemmy, I would probably extend the code a bit to support the categorisation that we needed. Have you had any interaction with the backend? I'm happy with programming Rust and JS so think it might be possible to add the features we need.
I haven't, and I haven't tried, but there are lots of new contributors over the last few months that seem to be picking it up fine. I'm not familiar with Rust so haven't looked at all.
Not really answering your question, but the biggest challenge with anything of this kind is getting people to move across. Facebook and the other big social media platforms are so embedded into people's lives, that they will protest loudly at being asked to move, even if what they're moving to is substantially better.
Lemmy is a pretty good platform, although still has a lot of holes. Over time, I think these will be improved, but that could takes years. For what you're talking about, have you considered just a standard, old-school forum? There are plenty of options here, but it seems like federation is maybe not that important to you. You just want everyone in one place, and one you can manage yourself. Old-school forums tend to suit that kind of discussion among smaller groups better than Lemmy/Reddit, which is more designed for larger numbers of users. There are even some that are planning integration with the fediverse if you did want that, although AFAIK none of them have actually done this yet.
If being on the fediverse was important, then Lemmy wouldn't be a bad option. With a few tweaks (e.g. disabling downvotes?) you could probably get it working for your needs.