Good resources to find the best niche to market in?

Hey guys, first post, i'm new to the game.

I'm currently growing ig pages related to different niches (mostly fitness and photography). I'd like to monetize em at some point but tbh I'm not confident in my niches, which means:

1. I'm not sure the niches I'm in have the potential to sell products and services in the future.
2. If they are technically profitable, I don't know for sure if they are oversaturated or not (and if it matters). (E.g. there are a billion fitness pages on IG out there, so maybe the competition is so strong that i will need thousands of dollars in ads to compete compared to other niches.)

So to make it simple, what are your methods / resources to increase certainty that your niche will be highly profitable once you have enough traffic? Are there any websites that rank niches in terms of the balance between profitability VS saturation?

Thanks and apologies for the newbie post, I hope it is still relevant and can help others out!

fitness niches will never get saturated, young people are always talking about making big muscle and many others are talking about losing fat, its a best niche i think, i love gym , i am always searching about things releted to fitness, people will buy it, just give them good offers like 20% off etc, and if you have knowledge about fitness then start a website write articles and send your ig traffic to website it will work bro

I'm not into growing IG pages but a wise lesson I have learned in relation to niches is that you have to be specific.
For example if you're in the fitness niche

Fitness > Fitness for man > Fitness exercises at home for man

In this way you're in a niche that isn't over-saturated and it's more profitable because you have a targeted audience.
Of course you have to do your own marketing research, this is just an example.