Alternative to a multistate object would be to use a Show/hide workflow. Sorry, will not write in German, because there would be too much 'Fehler'.
I suppose the last state (Erledigt) is a custom state, because you didn't like the InBuilt Visited state and want to have the custom state appear later. It would replace the Normal state in that case. Will that button still be kept 'active', can it still be clicked? If you disable the button, you could keep a transparent state and add the command 'Show' to show an image mimicking the Done state. Such an image can have a tag to be read by the screen reader.
Please, next time do never attach screenshots but insert them! I rarely dare to download attachments for security and time reasons. I have to download, open them in a graphics application (PNG is linked to PS in my case), view them, then delete them again from my system.