You could just kill Bismarck off. That would do it.
If you do that, do you get a German Empire at all? A non-Austrian one I mean.
A fair chance you might, but if the point is that Bismarck is an Irreplacable Man whose absence would mean no one smart enough to do as he did OTL was running things when the political crises hit that persuaded him OTL to go for mildly progressive reform, then presumably he was just as irreplacable back in 1871.
I guess you might mean he pulls off German unity, and
then dies. But why would his replacement(s) be persistently stupider than he was?
A more solid POD would be to have some good reason for Bismarck to decide not to do the reforms, and carry that off without the Empire getting torn up by the immediate crisis. And none of his later successors do them belatedly before things blow up completely. For that you need to research the detailed political history of the Second Reich, and consider the repercussions of this divergence on how that might evolve. Including management of other crises that would arise in the interim without this safety valve.
Without the reforms, I guess Germany has to in some combination get very lucky with economic developments which is pretty unlikely given that the whole world was plunged into depression at least once (I'm guessing, the very crisis Bismarck faced for this POD, the long crisis of the 1870s and '80s) and on a lesser scale more often than that. Or be more repressive, and somehow successfully so.
A more repressive Germany that nevertheless manages to do better economically at least on the domestic front will probably be that much scarier from a British point of view. Almost certainly that combination would result in bigger military buildups sooner, especially of the Fleet. OTOH I can sort of see how the German HSF might get built sooner, but the form the British panic takes is not yet to seek alliances against Germany but simply to rebuild her own navy, with the outcome that Britain's navy is more modern than the new German one, which either has to be rebuilt from scratch all over again (thus slowing them down) or is less threatening on the high seas than the later one OTL. Find the right mix and despite a stronger Germany Britain is not on the whole more panicked than OTL and so the Great War starts more or less on schedule.
France hates and fears Germany regardless of what they do, of course. I don't think a more autocratic Germany would have substantially different relations with Russia. The French will still seek to win over Russia by offering lots of loans for industrial development as OTL.
Then again, some accounts give the impression that Kaiser Wilhelm's firing Bismarck and then proceeding to mess up the good relations with Russia the Iron Chancellor had so carefully cultivated was an egregious act of sheer stupidity. I am not sure, I tend to assume things happen in history for good reasons more or less and that there were deep factors tending to rip the alliance of eastern autocrats apart.
Hey, if Wilhelm really was that much of a willful idiot, perhaps Bismarck does institute the reforms--but then the new Kaiser maneuvers them out again at the same time as he is mucking up relations with Russia? Say in the interim conservatives have been more unsettled and outraged than OTL at the same time as the economy in general has been doing better and working-class politics is weakened. (Actually a stronger economy would probably tend to strengthen working-class movements--so say they have been so relatively successful, this feeds the outrage of the better off, including a lot of middle-class types). Wilhelm cracks down, including taking the already-established reforms back off the table. The workers are the ones outraged by now but various forces of repression break their movement, and their wages stagnate or even decline even as German industry continues to grow, so the profits accruing to the better-off classes are downright obscene. And they don't want to give up a pfennig of it! And so Germany struts maniacally toward the Great War with a bloated military-industrial complex, a more blatantly anti-democratic political order, and a sullen but cowed working class smoldering underneath.
There are no Social Democrats in the Reichstag to vote the war credits; that's all done by the conservative minority allowed representation. Individuals who are more or less supporters of Social Democracy but don't dare say anything about it get drafted or volunteer to join the armies when war is looming, they serve loyally and well, but when things go south, SDs are in no way to blame for Germany's disasters and appear that much more appealing to broad sectors--including some of those middle class types who disdained the masses a generation or two before, and who knows, maybe a lot of industrialist types who see a rationalistic Social Democracy as a new lease on life for their industrial schemes, if only they can win over the Socialist leadership to keeping them on as design bureau heads?