Archive of Our Own absolutely dominates this pairing. The tag system makes it so easy to filter for exactly what you want—established relationship, post-war, slice of life, you name it. You get a lot of thoughtful, character-driven stuff there, people really digging into how their political marriage actually functions. Tumblr still has a vibrant community too, but it's more snippets, headcanons, and shorter fics, which is fun if you're just scrolling.
Honestly, I burned out on FanFiction.net for this ship years ago; the search is a nightmare and so much of it is older, trope-heavy high school AUs that feel dated. The real meat is on AO3 these days. I've even seen some fantastic, novel-length ShikaTema fics pop up on SpaceBattles forums, of all places, usually crossovers with other 'tactical genius' characters but still heavily focused on their dynamic.
Sometimes I think the quieter, more pragmatic pairings like this one just thrive better on platforms built for deeper dives.