According to the Wikipedia article on the proposed Istanbul Canal, it is said that the Bosporus has nearly 3 times the amount of ship traffic as the Suez Canal. Presumably this ship traffic comes primarily from Russia and Ukraine. Why does Russia choose to be so dependent on a water passage that it does not control? Why hasn't Russia (and historically, the USSR) tried to orient their economy to use the Baltic Sea instead? Are there economic and political obstacles to minimizing this dependence?
Because the Baltic Sea ices over every winter. In theory, you could continue year-round operations with icebreakers and cargo ships with a sufficient ice class but that's expensive and there are only so many ice rated cargo ships out there. Alternatively, you could use the St. Petersburg port heavily during the summer months and the Black Sea heavily during the winter but then you'd need all of the infrastructure at both ports which, again, gets expensive. Plus, in order to get to the ocean, Russian traffic would still need to go through a relatively narrow straight owned by foreign powers (Norway, Sweden, and Denmark in this case).
There is common wisdom from Western academics and politicians that the drive to own warm water ports has been a key driver of much of modern Russian history precisely because sea traffic into and out of the country is subject to such external bottlenecks. Not everyone agrees with this thesis but it is pretty common thinking in political science circles (and there are plenty of memes available if you do a search).
According to Montreux Convention Regarding the Regime of the Straits, Turkey is obliged to provide free passage of civilian vessels in peacetime.
Of course, there is a small possibility that Turkey won't honor the agreement for some reason, but given that such access is by no means existential for Russia, the problem doesn't seem to be serious enough to try to do something to solve it.