[{"_id":ObjectId("6347e5890c009a37b81da701"),"testField2":"2000","timestamp":ISODate("2022-10-15T11:00:12.000Z"),"testField3":NumberInt(2)},{"_id":ObjectId("6347e5aa0c009a37b81da700"),"testField1":"1000","testField2":"2000","timestamp":ISODate("2022-10-15T10:00:12.000Z"),"testField3":NumberInt(1)},{"_id":ObjectId("6347e5960c009a37b81da702"),"timestamp":ISODate("2022-10-15T12:00:12.000Z"),"testField3":NumberInt(3)},{"_id":ObjectId("6347e5960c009a37b81da703"),"timestamp":ISODate("2022-10-17T12:00:12.000Z"),"testField3":NumberInt(3)}]

db.collection.aggregate([{$facet:{op1:[{$match:{"testField1":"1000","testField2":"2000"}}],op2:[{$match:{"testField1":null,"testField2":"2000"}}],op3:[{$match:{"testField1":null,"testField2":null}},{$sort:{timestamp:-1}},{$limit:1}]}},{$project:{res:{$ifNull:[{$first:"$op1"},{$first:"$op2"},{$first:"$op3"}]}}},{$replaceRoot:{newRoot:"$res"}}])