Document a repeatable process to size online audiences and provide 5 example groups of people who are deeply passionate about a genre

Hi, I'm looking to create a product to sell online, but first I need a repeatable process (best practice process) to size various markets to help guide what I should create (fish where the fish are). The process could involve keyword search volume, size of Facebook or Instagram groups, social media buzz/comments, purchase intent etc, including what are the best tools to use. For example, there is a Facebook group of 421k members interested in "Art Deco" which indicates a passion for this area. I want to find the biggest online 'passion groups' to create a product one of these groups would love (and buy). It's important the process doesn't just look at audience size, but also an opportunity to create a relevant product to sell i.e. "Taylor Swift" may have a huge associated audience but due to copyright, it would be difficult to create a product to sell. There must be a filter in the process that includes realistic 'purchase intent'.
Once you have detailed the ideal process to uncover the largest groups, please list 5 identified groups using the process, detailing each of the metrics and tools used.
Thanks,
David