Kazan is much more interesting then Tyumen and Omsk. You should definitely spend more time in Kazan. Try to make at least one night stopover at Nighny Novgorod. Reduce nights in Omsk and Tyumen. If possible make a one-night trip to Tomsk from Novosibirsk (one night in Novosibirsk is enough too). Bare in mind, around large cities you planned there are many small interesting towns and places to visit.