I decided to give The Promised Neverland a try after all. It's really good; very engaging, imaginative story and good characters (and not too much horror, thankfully, because that's what held me back before). Love the moral dilemmas, too.
Demon Slayer on the other hand still fails to hold my attention. The story is passable (nothing great, nothing awful), but the MC is just so... bland. Like, I know he's strong, brave, kind-hearted and determined - but that's all I can say about him. That description could apply to any MC in Jump! His sister is even worse, all we know about her is that she used to be really nice. Some of the side characters are somewhat better, but they never stay around for long, and then it's back to Dull and Duller. So yeah, if I still don't care if these people live or die after 60 chapters, it's probably safe to say that my initial judgement was correct and the story isn't for me.