This is a poor decision by the company's management. I'd first reverse this decision because I don't think they can afford it. You should first tell management

This is a poor decision by the company's management. I'd first reverse this decision because I don't think they can afford it. You should first tell management that their expensive and unrealistic expectations (including the zero-detect business model), certainly without matching support for you and the team (hire more skilled team members, resources, understanding, time, accept delays in deliverables, ...) is not the right way to go about this or realistic. With that out of the way, in terms of improving the code and platform, take your wins where you can get it, it will be a gradual process. Do you know what parts of the system have the most defects and high severity? Adding a unit/integration testing framework, along with build automation that runs the tests, and training in the team about the benefits and how to write a test, would probably be my first stop. You could then start to add tests (and would need some code refactoring as needed) to the high-defect parts of the code to start adding some defenses. If this is a web based app with no framework, using a browser-based tool to run web based regression tests may be appealing too (see https://reflect.run/ https://reflect.run/ - I saw it mentioned on HN recently).