This is false. Kazakhstan retains most oil revenues and its joint ventures with foreign companies for oil exploration and drilling include not just Western, but also for example Chinese companies.
>Why Nazarbaev had replaced Cyrillic alphabet with Latin?
Because he saw clearly how Putin is weaponizing language and identity rhetoric to justify wars of aggression and Anschluss-style annexations against its neighbours. The script you use is rather symbolic, but those symbols do matter. Otherwise, Russia itself wouldn't have stamped out the Latin script and forced those minority languages, already few in number, to use Cyrillic in the 2000s.