I tend to rely on Archive of Our Own because the tagging system is unbeatable for finding specific dynamics. You can filter by relationship tags like "Peter Parker/Gwen Stacy (Spider-Gwen)" or "Gwen Stacy & Peter Parker," which really helps separate the Earth-65 stuff from the main 616 universe stories.
The best stuff often has tags like "Identity Reveal," "Canon Divergence," or "Fix-It," which signal a more plotted story rather than pure fluff. Some writers on there are doing genuinely ambitious things, like crossing over the Spider-Verse event with the emotional beats of the 'Into the Spider-Verse' film. I'd also suggest checking the bookmarks of authors you enjoy—it's a solid way to find stories they themselves think are top-tier.
Honestly, the quality on AO3 feels consistently higher than some other spots, maybe because people put more care into the tags and summaries. I've stumbled upon a few long-form fics that explore a post-'Across the Spider-Verse' scenario where Miles and Gwen have to rebuild the multiverse, and they're shockingly good. The comment culture there encourages deeper discussion, too.