[{"_id":1,"images":[{"name":"img0","order":0}]},{"_id":2,"images":[]},{"_id":3,"images":[{"name":"img0","order":3},{"name":"img1","order":5}]},{"_id":4}]

db.collection.update({},[{"$set":{"images":{"$let":{"vars":{"seq":{"$ifNull":[{"$add":[{"$max":"$images.order"},1]},0]}},in:{"$concatArrays":[{"$ifNull":["$images",[]]},[/**your payload here*/{"name":"new image","order":"$$seq"}]]}}}}}],{multi:true})