I would say you're being overly cautious.
Tapas does not care about swearing (unless it's like hate speech, obviously). So the only time I personally would mark an episode as mature for swearing is if slurs are included in the text. Slurs that target a minority or marginalized group.
As for violence and disturbing imagery, pretty much anything goes for novels. You could tag it mature for the sake of your audience, but Tapas isn't bothered by it and won't dock you.
As for sex, you can get away with a lot in a novel. I only tag mature if it's actually describing penetration, genitalia, and the like. If it's just fade to black or simply focuses on emotions, it can pass without the tag.
And if you're worried about setting up your audience's expectations, you can simply add a content warning to your series description or as a disclaimer ahead of your first episode.