I recently heard a talk by a developer who inherited a similar situation when he became the sole developer at a company. The codebase was a large blob of code. Git had only been used once for a single commit long ago. And testing was done via a sandbox folder on the live server. He took a microservices approach to incrementally improving the code base. Basically he pealed off discrete business functions which needed improvement, or created new verticals in the code for new functions, one at a time. And kept the code base as modular as he could. He gradually replaced the code base over time rather than rewriting everything at once. Some of his advice included things like resist the urge to rewrite code that is working. Use third-party solutions for business functions when it works. He used Square Space for the home page/marketing pages, in part to let the non-developers make copy changes while continued to work on the custom code. I think you are probably in a pickle though without the help of the rest of your company. Have you spoken with your senior developers and managers? Do they recognize a problem? Are they open to solutions?