Claude setup for Bog projects

Hey, I have been working for a year on my business, I started with Gemini, then moved to Claude code some months ago. I am on max X5 plan. The project is big, 100s if not more files, 100 of thousands of lines of code, custom website, custom payment, custom plugins, custom services, complex databases etc.. all working together. I use a mix of Claude Fable 5.1 and Opus 5. But I am seeing big issues with these 2 models, they are fixing bugs, breaking other stuff behind it, looping the same bugs. It's reasoning on some architecture is very "dumb" and makes a lot of mistakes, also it uses lately a lot of usage. For the architecture of the code and company I am of course always there to make the bigger decisions to steer it in the right way but it seems overwhelmed maybe by the amount of files and code and it cannot handle it? So my question is, in my situation, is Claude code really the best way to go forward? Is there maybe a better setup (proven) that could make Claude "code" and "think" better? All while using not so much usage? Because for now the memory.md is very limiting. Many thanks, Sam.