Building a CMS isn't lean. Using an off-the-shelf CMS that is selling enough of your photographers' work that both you, the end customer and the photographers all are saying isn't working anymore because of X, Y and Z reasons is lean.

Then, if you're at that point, I'd start looking at hiring a full-time dev to build a product infrastructure that's going to scale you up, instead of outsourcing.

I've been involved in too many "we'll just build the product with a custom version of an existing CMS" and also the "we're going to build everything from scratch before knowing what anyone really actually needs" so hope you can avoid some mistakes I've seen and felt pain for!

Happy to talk if I can be of help.