Hire a senior engineer. They would probably 1) put the whole thing into version control, 2) streamline the deployment process so that specific versions can be p

Hire a senior engineer. They would probably 1) put the whole thing into version control, 2) streamline the deployment process so that specific versions can be pushed easily into a sandbox or production environment, 3) begin writing tests, starting with end-end and system tests in this case, and hook them into a continuous test harness 4) do an architectural review to identify major components that can be split off from the monolith in order to reduce the surface area to work off, start with the part that you will get the most bang for your buck, a business critical area for improvement. 5) add unit tests and integration tests for this and the main component. 6) Repeat as necessary. I don’t believe this is a problem that can be solved with people skills alone. It requires senior technical expertise.