In response to the Slime, YES, DO IT
As for the Japanese stories you mentioned, I know Overlord is popular because of the anime and stuff, but I haven't come across anyone else who knew about Realist Kingdom!!! OMG I love it so much. The harem bit is...eh, typical, but the trashiness is addicting so I can't help myself. I also find it somewhat refreshing that the harem don't all act aggressive, hateful, or b*** to one another. There all a big family, albiet a bit complicated, and it makes my heart feel all warm in fuzzy. Especially with one particular family developmemt that has taken place in Vol. 7 & 8, but for spoiler reasons, I'll say no more.