I would potentially pay for this in Mandarin, although I'd be wary of the quality vs human-produced sources.

I often practice by reading sites like https://cn.wsj.com/, https://www.bbc.com/zhongwen/trad, https://cn.nytimes.com/, or https://big5.ftchinese.com/. These are all text news sites so they help with reading comprehension and grammar but not at all with listening. I'm not aware of any similar audio sources, especially not ones that use professional-level language but slow it down so it's accessible to language learners. So you'd be in a unique niche.