@Lukeliush Could you share some of the possible storage area layouts that your 3rd party warehouses are using? Also what kind of products are you dealing with? Consumer electronics? Auto parts? Household goods? Grocery items? Do these items expire? Do they require cold chain? Is there an even distribution of products represented in orders? Do you good data about the distribution and volume of items in orders?
Here’s an example of a commercial system that supports storage layout planning and optimization.
https://www.infor.com/products/supply-chain-management/warehouse-management
Here’s a case study for a simulation software called Anylogic that does something similar to what you want to do.
And here are some public models produced by Anylogic. I couldn’t find an order picking example but you can get an idea of what the software can do by perusing these examples: