Ao3 is pretty much the beating heart for that specific niche. The tagging system lets you filter for things like 'Harry Potter/Harem' or 'Harry Potter/Multiple Partners' with a precision that other sites just can't match. I've found some truly epic longfics there that explore the political or magical ramifications of those relationships, going way beyond just wish-fulfillment. The community tends to skew older, so the writing quality is generally higher, and there's less of a push toward the same repetitive tropes you see elsewhere.
That said, FanFiction.net still has a massive back catalog. A lot of the classics from the mid-2000s, the ones that basically defined the 'Harry has multiple soulmates' or 'Goblet of Fire champion inheritance' tropes, are still there. The search is a nightmare, though. You're better off finding recommendations on forums like SpaceBattles or Sufficient Velocity and then hunting down the titles on FFN. Those forum threads often have passionate debates about power dynamics and plot logic that are almost as entertaining as the stories themselves.
Surprisingly, some of the more interesting takes pop up on Questionable Questing. The site has a reputation, sure, but the 'Creative Writing' section attracts authors who want to blend the harem element with genuinely creative world-building or crossovers, often with a darker edge. You won't find it on Google easily, which keeps the discussion a bit more focused. I stumbled onto a fusion with 'The Dresden Files' there that was shockingly well-researched.