Let's say I'm targeting 2 words keyphrase but I made additional articles for subpages (related to topic, of course). I wrote 10 articles with about 500 words each and made 10 subpages- one for every article. Articles contain main keyword + one more word. Now I need to create index page. I'm planing to write 300-400 words article for index and copy about 200 words from every subpage so the index would look well organized, more attractive and to gain some seo value for targeted keyword if possible. Is it a good idea to copy text since it is topic related and helps visitors?
What you want to try is good for you but other way. It is a good strategy to put some introduction paragraphs for every article but don't copy them. Try to rewrite them, let's say, like little introduction text. It will not harm your website if you copy 100-200 words and put on your index page but it will have benefits if they are changed, not only copied. Also, you can make a little bit longer article for that index page, than 300-400 text because that will be the main content to target, and should have higher %.