It isn’t really a problem of being cross platform: I don’t think platform UIs have enough richness to handle such applications without a lot of non standard components. You can only do so much with sliders.
Likewise, accessibility capabilities rely on very standard generic UI capabilities. Once you go beyond that with professional tooling, accessibility will have to be custom as well.
I'm not convinced that standard controls are insufficient for a DAW plugin UI. Can you please explain why? Is it all about looks, or are there actually usability problems for sighted users?
Let me flip that question around: given what the plugin UI does, how would the interface actually look using LCD UI components and would the experience even be meaningful for unsighted users?
Real time scrubbing and mixing with...a screen reader...how does that even work? I’m sure it could work, but you would still wind up with something as custom as seen here, perhaps even more so. Accessibility isn’t a free lunch except in very generic limited use contexts.
Likewise, accessibility capabilities rely on very standard generic UI capabilities. Once you go beyond that with professional tooling, accessibility will have to be custom as well.