Looking for mature Thalia/Reyna fics that aren't just pure, unadulterated smut? That's a specific ask, but I get it. You want the tension, the angst, the complex history and power dynamics of two daughters of warring pantheons, without the story being completely derailed by porn. My top recommendation would be to hit Archive of Our Own and use the 'Mature' rating filter, but then also check for the tags 'Slow Burn', 'Emotional Hurt/Comfort', or 'Post-BoO'. You can exclude the 'Explicit' rating if you want. There's one called 'The Unkindled Coast' that sticks in my mind—it deals with their respective legacies and the weight of leadership after everything settles, and the romantic tension is there in the quiet moments, not just in bed. It felt true to their characters.
A lot of the safer mature content explores their shared isolation, the burden of command, and the what-ifs of a very different life. Sometimes filtering for longer fics helps, because those tend to build the relationship more fully. And honestly, don't sleep on Tumblr blogs dedicated to Percabeth-adjacent ships; some writers post really thoughtful, rated-T multi-chapter pieces there with links to AO3. Just be prepared to wade through a lot of headcanon posts first.