IIRC the SRDC was included in the special edition release. So probably a DLC? If they add new images and some 2017-ish goodies then I don't mind paying for a DLC.
If there is something I want that does not require much work from the HD version, it's the HD version of the SRDC with the English translations of the short stories.
You can read the side story #2 if you want to learn more about fal, outside her route. Side stories #1 and #3 deals with the rise of Phorni and her Piova adventure and the reason as to why Phorni is a flightless fairy respectively. The other are not translated but you can read them in Japanese from the Digital Art collection.
IMO, the reason the 3 main route are kind of meh/dull is due to the fact that the character perspective remained with Chris. There are certain parts of the story that felt rush when you read it but you'll realize in Al Fine that everything actually makes sense. About the last route
He couldn't get Lise from Symphonic Rain, although she's in his database. Akinator needs more training. Considering she is the weakest character/route, i doubt a lot of people though of her. It's like the 1% or 1%.