Hi! Kazan is a great city. Although I live in Moscow, I have been there many times. This is a great place. There are a lot of beautiful sights and by the way, I advise you to visit the water park because it is simply gigantic there. And at the expense of documents, I think that there should be no difficulties, maybe you should issue an invitation letter russia using the website https://www.russia-travel.com/visa-services/ or I think that it will simply be easier for you to go to the site and turn to professionals for help.