Sadly I have a similar background; data analytic company where the entirety of the MVP was built and maintained by a two person shop in Detroit. Our company had become so dependent that when the initial contract was up, they wanted a renewal of $1mil... and we gave it to them...

On the bright side, I learned a lot about building and working on RESTful APIs during the re-negotiation period where I might become the new developer

At one point, a lot of my consulting business was figuring out code like that, that others have written.