I have changed languages, tools, domains many times in my 40+ year career. Freelancing for the last 15 years I have to quickly jump into a code base and domain

I have changed languages, tools, domains many times in my 40+ year career. Freelancing for the last 15 years I have to quickly jump into a code base and domain and deliver results. I started a long time ago with BASIC and FORTRAN, wrote a lot of COBOL, then a lot of C and assembly language, then C++ for a long time, then I moved into web development using all kinds of tools. I have never found it all that hard to learn new languages and programming tools. As you note learning business domains takes longer, but over time you get better at it because the knowledge at least partially transfers. More than mastering a particular "stack" I recommend getting good at listening and asking questions to understand the business domain and the legacy code landscape, and getting effective quickly with unfamiliar languages and tools. Complete mastery takes a long time, but usually you don't have to understand every detail of a language or platform to fix problems and add value. Focusing on one or a handful of languages/tools and labeling yourself that way turns into a self-limiting box. Employers and customers pay for solving business problems with programming skills, not for my deep understanding of C++ or Linux.