Mastermind Bismarck is a legend he himself created in his memoirs. Bismarck didnt even have an united Germany as aim in mind. What he wanted was an uber-Prussia, and he got that in the form of the NGC. Further German unification was not really planned, and partly came about as result of the Franco-German War, not the other way round. Thats how I see it at least (following Haeffners intepretation).
That is a valid view about Bismarck. But if the NGC is the final aim of Bismarck's ambition for Prussia, I see no point in provoking Napoleon III at all. The only other explanation were.......Bonapartism.
And (small-) German unification
was planned on level after level- maybe not by Bismarck.
Small-Germany as a Super-Prussia had been on the screen as a faint possibility since 1849/1850 (Erfurt).
But especially after 1866, it was on its way. There was the NGC, then the Zollverein, which encompassed almost the complete small-Germany by then. The latter turned into a kind of "secondary NGC", since 1868 there was the "Zollparliament" (as an expanded NGC-Bundesrat) and member states gave up the right to veto common decisions. A currency reform was under discussion. <br>
Since 1866, all small-German nations were tied together in alliances under leadership of the Prussian king, most states forming common structures.
At the outbreak of the Franco-Prussian war, the German Reich was already in formation, economically, politically, militarily - the process is IMHO comparable to the USA in the 1780s or the EU.
If Bismarck didn't want anything more than an NGC-arrondated Prussia, he would have had to actively work against it. Even if I concede that unification wasn't his ambition (it certainly wasn't King Wilhelm's), his policies ensured that the process went carefully, step-by-step and Prussia-dominted, taking the least international risk by avoiding revolutionary acceleration of the process or wars where there would have been any doubt about the defensive role of Prussia & Co.