Do not waste your time reading this manwha, everything you need to know is contained in the synopsis. Yes they kill the Demon King, all the hints of future tech in a fantasy world? They mean nothing. Every action they take after killing the Demon King means nothing, the MC undoes it all with time travel in the last two chapters and returns to a ruined earth. There are no cool fights, there's no feeling of progression, nobody develops, the interesting characters (the Hero and Saintess) get killed off less than halfway into the story.
Read something else, do something else, take a nap, this is not worth your precious time.