Really interesting story, thanks for sharing with us!
I am doing a lot of logistics and robotics automation and you have reached a very efficient environment with as it seems just a moderate investment. Congratulations with that.
I understand g1smd's comment about monkeys. One of my customers 20 years ago declared that he only wanted monkeys which where able to push a green button. Actually after a lot of automation the contrary happened. Mechanical and automated systems were taking over the boring and repeating work while the people were still needed for the jobs where a flexible brain was useful. So instead of creating monkeys, they created a flexible work force which was able to supplement the automated systems which were doing the bulk of the repeating work.
You can do the same type of improvement here. As your warehouse will be getting larger, much more time will be spent on walking/transport between one location and the other, and humans will become increasingly less efficient in your setup if the warehouse size increases. The next step I would therefore recommend is not adding the flashing light type of thing, but adding mechanized transport from your storage locations to the packing location. In that way the picker can dedicate his full time with picking products, while the transport task is taken over by something as simple as a conveyor belt. Not a huge investment in terms of money or automation, but depending on the warehouse size it can give a significant improvement in the picker's duty cycle.
Another optimization point which won't cost you any money, just time to analyze your order data, is to cluster the products in such a way that items which are ordered often together are in the same warehouse region. Many e-commerce websites offer the "other customers also bought" lists which give an indication of which items are often ordered together. Average walking distance per order can significantly decrease if you optimize your product locations based on that data.