Another aspect here is risk. One of the risky portions of development is already complete, the development phase, that is, your engineering design, prototyping, agency certification, etc. The future manufacturer of your product has a reasonably low risk, assuming he has properly checked your credit and references, and delivery and final payment are linked in your contract. The various warehousing and order fulfillment services, too, are not terribly risky, and they also will typically hold your inventory. But the remaining risk is marketing and sales. One might have a great product, well manufactured, with full warehouses ready to ship. But if the marketing is wrong, or the timing is wrong, etc, it might not succeed in the marketplace. Asking someone to take on marketing and sales of a product is a big deal, and I'm not surprised the few companies that do this demand a very large percentage of the profit.
As engineers, we tend to focus on the importance of the design, manufacturing, etc. And I know that in the past, I have looked down on the sales side as inferior and just a nuisance. But working on several consumer and industrial projects has given me a newfound, albeit begrudging, respect for those folks. I told myself, their world is completely different from mine, and at times I almost loathe them, but they bring a needed skillset to the table and are necessary cog in the machinery of a successful product.
By the way, one phase that might need some additional work is adapting the manufacturing process you developed, presumably without any specific manufacturer's input. Depending on the complexity of your assembly and testing, there will be some additional tailoring of your procedures and test equipment to the selected manufacturer(s). This isn't usually risky, but it can take some additional effort.
I've been through what you're facing, and even dabbled with the marketing and sales, and can appreciate the issues you're facing. Best of luck to you.