What you describe is controlled capitalism. People can decide themselves what they want to do and try to get things done in the most efficient way directly without government interference.
The problem current capitalism faces is that there is too little control, too much allowance for monopolies, that sort of shit. Tax the crap out of the rich, limit what you can do “if you create polluting materials, you have to recycle them yourself”, “you cant corner more than 10% of a market”, etc, but allow people to freely do what they want to do. That would be capitalism, actually.
everything necessary for society to function would be taken care of during the said 20 hour workweek
Yeah that is not how society works, that is not how anything works at all. You don’t work 40 hours a week just to make somebody rich even richer. If they could pay you only for 20 hours, they would. You work 40 hours because you CAN have a job which is because they need somebody to do that work. If they don’t need you, they won’t pay you for nothing dummie. If you work on something not required, congrats, you have a dumb boss that wastes resources and you lucked out. Most people just have normal jobs that NEED to be done. Just saying “lets do communism and we only work 20 hours a week” is beyond naive. Reality is “Lets do communism and half of us will starve to death!”