So the main reason that I chose to read this is so I could finish reading it before the English translation comes out in a few days (and also because I like the franchise). Also possible spoilers for people who don’t know what happens in the original Steins;Gate (not that anyone hasn’t seen Steins;Gate anyway but just in case).
Chaos;Child is the sequel to Chaos;Head, with the story dealing with the aftermath of the events in Chaos;Head. It is also part of the SciADV series just like Steins;Gate and Robotics;Notes.
Dies irae is one of the visual novels that is quite highly regarded in English speaking circles despite not having any English translation available yet. (Although there is an English translation being currently worked on so hopefully that changes soon.)
Akatsuki no Goei ~Tsumibukaki Shuumatsuron~ is the direct sequel of the original Akatsuki no Goei, with a more focused plot building off the setting and all the foreshadowing in the previous 2 games.