Consider a query like:

[
    { $lookup: {
        from: "b",
        ...
        as: "y"
    }},
    { $lookup: {
        from: "a",
        ...
        as: "x"
    }},
    { $unwind: "$y" },
    { $unwind: "$x" },
  ]

Recently I asked Claude to generate a query, which actually ended up being near join-eligible but wasn't because of the placement of the $unwinds. I suspect this would be a beneficial rewrite regardless of join-optimization. Example prompt/query in comments.