It depends on whether you're looking for something that's more commercially published or deep in the community spaces. A lot of the really compelling, character-driven taboo stuff isn't on mainstream storefronts for obvious reasons. I've stumbled across some incredible writing on specific subreddits dedicated to the genre, but you have to dig. The signal-to-noise ratio can be rough.
What makes a character 'strong' in this context varies wildly. Sometimes it's about emotional resilience in the face of impossible desire, other times it's about raw, dominant power dynamics within the forbidden framework. 'Taboo Tales' on certain forum-based sites often have longer serials where the author really fleshes out the internal conflict, which for me is the whole point. The sex is almost secondary to the psychological torque.
You might find some self-published authors on Smashwords who push boundaries further than Amazon allows, but the quality is a total gamble. I tend to follow writers, not platforms. Once I find an author who handles the tension and consequence with nuance, I'll hunt down their entire backlist, even if it means jumping through a few hoops.