I'm not sure there is much point in trying to detangle Bismarck's "German Empire" ambitions from his "pro-Prussian" ones; Prussian power was such that improving it led almost inevitably to Empire. Even the reversals and compromises alluded to above seem inevitable given the diverse social situation in Germany; he bloody well had to have made some compromise with liberalism, for instance, and cleverly did so in a fashion that surrendered a minimum of power. For one thing, as a federal union, the Empire was remarkably weak, having many parallels in its formal structure to the Confederate States; the real unity of the Empire was that the single kingdom of Prussia within it dwarfed the others--the "Empire" was indeed basically a sock puppet of the Prussian Kingdom. An advantage of this, from the point of view of a conservative authoritarian who dislikes liberalism but must tolerate it, is that its strongholds can be isolated. Another is that taking power on the Reich level by democratic means gave the democrats very little real power; to really run things they had to have leverage over the Prussian state. But meanwhile, Prussia had in its kingdom constitution an institution of reserving 2/3 of the seats in its own parliamentary legislature for the two classes that paid the upper two of three levels of a certain tax; these numbered far less than 2/3, or even half, of the populace permitted to vote. With profoundly illiberal institutions like that (one might argue that votes should be proportional to taxes paid, but actually the poorer Prussians paid a lot of other taxes not considered in assigning these parliamentary seats--much the same is true today of the US political charge that "most Americans don't pay taxes!"--they are looking just at income tax, not the whole broad spectrum of all taxes, which with a few exceptions like income tax, poor people actually pay a higher proportion of than rich) Bismarck was having his cake and eating it too.
For Prussia not to become an Empire in Bismarck's time, either he or some alternate chancellor would have to really drop the ball a lot, simultaneously blowing Prussia's chances for imperial supremacy and hurting Prussia's status as a separate kingdom. Conceivably if some alternate version of himself or some other person had been Chancellor who had some idealistic vision of Germany united but not under Prussian hegemony, or had the notion that Germany actually shouldn't be united at all, policy might have been different--but it's hard to see how Prussia would not visibly suffer, and Prussian ruling circles would rightly, from a Prussian point of view, sack such unpatriotic visionaries. Or there could be someone with Bismarck's goals but not his skill, who just blows it.
Still I suppose there is some value in asking the question--it serves to remind us, Bismarck did not know for sure he'd succeed in unifying Germany, and probably did not think in terms of that as a definite and certain goal for most of his pre-1871 career; he was indeed thinking of Prussia first and the possible goal of a unified Germany under Prussia was very much secondary.