But all free sites are working for a limited periods only. Also all url doesn't contain the relevant information of the websites too
It's the question many people ask – can you do SEO yourself? You can absolutely do SEO yourself. With some research and lots of practice, anyone can learn how to do SEO for their business. A quick way to get started with SEO is to enter your URL here and then focus your SEO efforts on the recommended action items.
Surfer SEO can do something similar in their Content Planner with a domain level search. Not 100% what you are looking for, but pretty darn close I suspect.
INSTANTLY ANALYZE YOUR SEO ISSUES FOR FREE Quick SEO Audit helps to identify your SEO mistakes and better optimize your site Content. Unlimited Analysis Run unlimited analysis on our most powerful servers. Stored reports make it easy to view progress and past work. In-Depth Reviews With our in-depth website analysis learn how to fix your SEO issues with clear definitions for each SEO metrics. Competitive Analysis Side-by-side SEO comparisons with your competitors. See how your SEO can improve against the competition. Click to analyze your site now: https://www.qseoaudit.com/
I think you are putting the cart before the horse. First you determine keywords for your page/product Then you start writing content using those keywords (and synonyms of those keywords)
There are plenty of site analyser on Google that can help you find keywords. Plus there are a few Chrome extensions as well.
I think tools like Surfer SEO could be helpful for this, I think they have some free plans and trial periods so that you can try it out and see if it meets your requirements.