I’ve found this official documentation which answers most of my questions about accessibility in Zorin OS:
I’ll keep this topic open for now, as I plan to install and test Zorin OS tomorrow.
If I encounter any accessibility-related issues during installation or daily use, I’ll report them here so they can be discussed and documented.
Orca is preinstalled on Zorin and available in the live session. You only need to go to settings > accessibility > seeing and enable the screenreader there. I'm not sure about the Zorin installer.
Edit: After starting the installer tool the screenreader orca stopped working. It seems not to be possible to use it during the installation.
Here you can see that it is installed in live session. It is not necessary to install orca.
Orca isn’t included by default in the live session, but once installed you can quickly enable it through Settings → Accessibility. GNOME features like font scaling and high contrast work without any issues.
When You need that, I would suggest to activate after the Installation in Settings>Accessability the Toggle for ''Always Show Accessebility Menu''. then You get in the Taskbar a little Human Icon. And when You click on it, You get several Option to activate.