While this is a good point, I'm not so sure that this is the case. The OP has over a decade of experience, but is noting issues only after deciding to work for

While this is a good point, I'm not so sure that this is the case. The OP has over a decade of experience, but is noting issues only after deciding to work for a "proper medium software company". It sounds like the OP may not have been used to working on projects that place a heavy emphasis on testability. But now that that's an important consideration, it's changing the entire approach for all of OP's implementations, which can be pretty startling. If the OP hasn't been writing working software for the last ten years, I'd say that it wouldn't be a case of imposter syndrome.