I stumbled into this niche by accident, and honestly, the best place to start isn't the obvious one. Archive of Our Own's tag system is your friend, but you have to get creative with the search terms. Try filtering for the 'Groundhog Day Loop' tag combined with the ship tags you like—Sasusaku tends to have a lot, but I've found some real gems for rare pairs like Naruto/Temari or even Kakashi/Iruka buried there. The time-loop mechanics often get really inventive, with authors using the repetition to build a slow-burn romance from the ground up, which I love.
A lot of folks overlook FanFiction.net because the filtering is so clunky, but I've had luck by sorting by favorites within the 'Naruto' category and then just manually scanning summaries for 'loop' or 'repeat' keywords. It's a pain, but sometimes the older fics there have a certain charm that newer platforms miss. My absolute favorite of all time is a Sasuke-centric one where he's the only one aware of the loop and has to win over a clueless Sakura—the frustration and eventual payoff were everything.
Don't sleep on dedicated forum threads either, especially on sites like SpaceBattles or Sufficient Velocity. They sometimes host collaborative 'quest' style stories where readers vote on the looped character's actions, and romantic subplots can emerge organically from that. It's a different flavor, more chaotic and community-driven, but you can find some truly unique dynamics there that you wouldn't see in a pre-planned fic.