How to charge for SEO research in addition to content writing?

I'm a freelance writer looking to start offering SEO services as an add-on service. Mostly just keyword research and content SEO, not technical SEO or backlink stuff (I can help my clients find guest posting channels, though and write content for those, though). Now the thing is, till now I have mostly worked as just a writer - which means my clients used to provide me with the keywords. So I just used to charge them on a per word basis. So wanted to check with any SEO professionals in here about what would be the best way to factor in the cost for SEO research if a client doesn't have keywords and wants me to take care of it in addition to writing. Should I add a flat fee or increase the per word rate? How much would be the ideal cost to charge for clients based in the US or Europe? Additional details, if they matter: I won't be doing the SEO research myself. My brother is good at SEO and we've previously ranked a couple of blogs we wrote as part of an experimental side project - he'll be taking care of the keyword research. As for the tools, we'll be using an Semrush subscription for now, can also take Ahrefs if needed. Plus other free tools, AI, and manual research, of course. Any guidance would be much appreciated!