I don't know whether it is more or less screenreader-compatible by design or how responsive the devs are about bugs. What I have seen is that people who hate Thunderbird's new layout in 115 strongly prefer Betterbird. It's a lot less cluttered and also seems to be snappier, though I haven't used it long so that could be inaccurate.

I have it running on my work laptop and unfortunately it inherits all of the Supernova changes, that is all of the inconveniences of the new interface. It's still F6 all the way!