this is called event sourcing and is quite difficult to engineer yourself the proper way. you'd be better off using Akka Persistence but the problem here is that this doesn't fit in your average server and actually forces you, to redesign your entire system to make fully use of that e.g turn every Player instance into an actor
not true. storing every event will definitely use more disk space
yeah sure you have your little client to server message pojo's but what you'd actually want is every tiny game event that happens (add item to inventory, remove item from inventory, take one step, subtract 10 from bank)