The manga wasn't so bad up until where it's currently translated (ch. 43?)
The WN on the other hand was slightly enjoyable at first, but it quickly turns into another generic WN. Boy is reincarnated > boy acquires power > school arc > boy ends up becoming hero > ???? I stopped there.
The reincarnation tag means very little and his time as king has almost no influence on his current life. He is portrayed as both mature and immature with the explanation being that all he did was fight in his former life so he isn't very emotionally developed.
What probably ended me annoying the most was the erratic pacing. There were times where the story was moving along so slowly I ended up skipping 'useless' dialogue, and then suddenly, out of no where, the story was forcibly pushed along by some drastic event.
Give it a try but don't get your hopes up.