ChorasDen, the norm for training and career development programs is three years per niche/role, which goes back in quality management IIRC to Taylor. Licensing stateside generally requires four years total experience and a senior attesting that you are competent in at least one niche, which makes sense given industry norm.
Working across multiple specialties simultaneously is usually called either being a generalist or a systems engineer and does not establish competence, hence the reason engineering training programs dont start recent grads in that role. Per the old stereotype, lousy generalists are the "jack of all trades master of none," emphasis on the later. Without deep knowledge of subordinate specialties, a generalist doesnt know how to correctly analyze their design or whether it meets requirements, and is left either guessing or committing the cardinal sin of blindly treating a "standard" as a "how-to" without understanding its context or nuances. To ethically and legally lead the design of complex systems requires quite a few years of experience or lying about one's competence, there's no shortcut otherwise. Many get away with the later in less technical/competitive/regulated niches bc their competence is never challenged by peers/competitors/courts. If you do so in highly technical/competitive/regulated niches without the requisite experience tho you'll be quickly called out, if not forced out.