I would like to display some actual temperatures, humidities, time and maybe some graph permanently on the wall. Does there exist some larger (like 12'') e-ink display for RPi?
All large e-ink displays I have seen were a mosaic of small displays. Manufacturers make medium to small book sized displays for example 13.3". Though it may be cheaper to get the parts from an existing consumer product if you plan a volume of one.
You can hook pretty much anything to the rpi but sometimes the work has already been done.
The largest ereader I know of is 10.2" and 10X the price of a 1080p 24" led monitor, so unless you are on solar it's economy a few years early for large e-ink.