There are so many barriers to this.

They need a venue at which to perform regularly, which helps somewhat to promote their performance. These are becoming few and far between. They are not likely to make much money for said performances (of course this depends on the size of the venue and the following they've managed to develop.)

Forgot about making any money selling recordings. People expect music to be free nowadays. The days of selling $10 cd's at your show are pretty much over. Maybe if you cater to an older crowd that isn't hip to the streaming/Youtube culture you can sell a few cd's here and there but certainly nowhere near enough to support yourself.

So... the "healthy primary income" you describe would be nearly impossible to achieve in today's environment. Most local musicians need to supplement their income working other jobs such as barista, Uber driver etc.

Definitely interested in this.

The musical skills might include reading literacy, proficiency in improvisational idioms and the ability to teach these things.

The more difficult realm might be establishing a working band of professionals, per individual the professional attitude would be worth more than sheer musical skill. Professionaly minded people working together can build skills that will eventually surpass what an individual can bring alone.

Networking is important, there's lots of ways to engage with your immediate community and it has always helped me to reach across as many professional aisles as I can. How many high functioning professional musicians do you think the average person knows? You should be one of them for the people close to you.

EDIT: I'd like to add venue versatility. Most discussion seems to be around entertainment venues but what about educational, accompaniment, theathrical, ceremonial, religious (I do not personally partake in this).