How to validate SEO website idea?

Hello everyone, I have read many online guides to SEO but I am new to this. I was considering making an affiliate marketing website that mainly uses SEO to generate traffic.

My idea was to do product reviews that would take a broad category of products and narrow down a large amount of choices to make the options more digestible. There is currently no review website for this particular broad category, but other large websites still rank for all keywords related to my idea. Free ranking checking tools indicate that my chosen short tail keywords all rank in the medium difficulty range.

What kind of signals would you look for to tell if an SEO strategy would be worth the time?

Would it be worth it to focus on the broad category even if I pursued long tail keywords or would it be better to narrow the focus of the whole website?

If I can clear up anything or provide more info, please let me know. Thanks in advance!