I’m a bit of terminal nerd, so probably not the best person to talk about desktop. I don’t have many thoughts with regards to app development or layout for accessibility. What I really would like is for distros to be accessible from the ground up, even before the desktop is up.

The best example of accessibility from the ground up I saw for linux was talking arch, an Arch Linux spin with speech. Sadly the website is gone, but we can find it in the web archive

• https://web.archive.org/web/20210116060142/https://talkingarch.tk/download.php

in particular there was an audio tutorial to help you install the live cd (you can still ear it in the archive):

• https://web.archive.org/web/20201206010519/https://talkingarch.tk/tutorials.php

Here are a few resources, which are pretty dated but I wish they were the norm in any install:

• your system can boot up talking, using speakup http://www.linux-speakup.org/speakup.html
• even if the desktop is not fully accessible or breaks, your console or the terminal in your desktop can speak using yasr https://yasr.sourceforge.net/

Now going into your points:

How should a blind Desktop be structured?

To be honest I don’t expect much here. As long as context/window switching signals you properly you are probably fine. I have not used gnome with orca in a long time, but this used to be ok. The problems begin with the apps, tabs and app internal structure.

Are there any big dealbreakers like Wayland, TTS engines, specific applications e.g.?

Lots.

Some times your screen reader breaks and its nice to have a magic key that restarts the screen reader, or the entire desktop. Or you just swap into a virtual console running speakup/yasr and do it yourself :D

TTS engines are probably ok. Some times people complain about the voices, but I think it is fine as long as it reliably works, does not hang, responds quickly.

Specific applications are tricky. The default settings on a lot of apps wont work well by default, but that is not surprising.

I do think that a lot of newer apps have two problems

1. they are not configurable or scriptable at all, there is only one way to do things and no way to customize it. Opening tickets to patch each and every feature is not feasible.
2. They frequently go through breaking release cycles that nuke old features, so you need to relearn all your tricks on the next major release and find new hacks

I can give you two good-ish examples, both Vim and Mutt can work very well with a terminal screen reader, but it is a lot of work to configure:

• with vim you need to disable all features that make the cursor jump around and draw stuff (like line numbers and the ruler)
• with mutt every single string in the screen can be customized, so you even insert SSML to control speech and read email

I think you can find similar examples in desktop apps too.

What do you think would be the best base Desktop to build such a setup on?

no idea to be honest. Gnome use to have support. I suppose other desktops that can be remote controlled could be changed to integrate speech (like i3 or sway).

Would you think an immutable, out of the box Distro like “Fedora Silversound”, with everything included, the best tools, presets, easy setup e.g. is a good idea?

I have never used Silversound. But the key thing for me is to be able to roll back forward to a working state.

How privacy-friendly can a usable blind Desktop be?

I think it should be fine. People with screens have things like those Laptop Screen Privacy Filter, people using audio have headphones. Depending on your machine you can setup the mixer so that audio never uses the external speaker.

I don’t recall the details but you can also have some applications send audio to the external speaker while others use your headphones (provided they are a separate sound card, like usb/bluetooth headphones).

Also, how would you like to call it? “A Talking Desktop”?

Urgh, Shouting Linux.