Your main problem is people don't like paying for anything, so the content has to be worth 10x-100x or alternatively just tell be what they want to hear with hindsight, or if you're really good then as it happens, no one is consistently predictive, if so the value is discounted 250x-1000x, yes that's how it works.
There are a couple of sites you can use that can then seed ideas for writing, I have access to a 3million word / 3,000 article finance and wealth management library written over the past years, 1 of the 100 books have been published using it

, on substack finance is not very high up in the top newsletters, try asking about their Substack Premium or Pro, they sponsor you to write in the first year.
AI and ChatGPT don't actually provide anything of value, it's just rat chasing rat, but this is the world today, most don't have any energy these days to create anything new so regurgitate what it provides, except AI has a problem with context, sure it will put most writers out of business but that doesn't make the world a better place, best to leave the minions with their tools and find a niche for yourself.
The Peach Pit and Doomberg are the top newsletters on Substack, but they are the minors compared to Gartman.