[{"_id":"001","Location":"NY","start_date":"2022-01-01T00:00:00Z","Foo":"fruits"},{"_id":"002","Location":"NY","start_date":"2022-01-02T00:00:00Z","Foo":"fruits"},{"_id":"011","Location":"NY","start_date":"2022-02-01T00:00:00Z","Bar":"vegetables"},{"_id":"012","Location":"NY","Start_Date":"2022-02-02T00:00:00Z","Bar":"vegetables"},{"_id":"101","Location":"NY","Start_Date":"2022-03-01T00:00:00Z","Baz":"pizza"},{"_id":"102","Location":"NY","Start_Date":"2022-03-2T00:00:00Z","Baz":"pizza"},]

db.collection.aggregate([{$project:{data:{$objectToArray:"$$ROOT"},start_date:{$ifNull:["$start_date","$Start_Date"]}}},{$unwind:"$data"},{$project:{start_date:1,key:"$data.k",_id:0}},{$sort:{start_date:1,key:1}},{$group:{_id:"$start_date",all_keys:{$push:"$key"}}},{$project:{all_keys:1,all_keys_string:{$reduce:{input:"$all_keys",initialValue:"",in:{$concat:["$$value","$$this"]}}}}},{$group:{_id:"$all_keys_string",all_keys:{$first:"$all_keys"},start_date:{$first:"$_id"}}},{$unset:"_id"}])