Okay interesting. By the time of the thirty years war, what do you mean by saying that any centralization would be hegemonic?
Rather than having an unified polity you have the Emperor as the top prince keeping the other ones down, something like Shogunate Japan and the Han system, of course said process would be less than stable and with France, Sweden, the Netherlands, etc creeping in it creates a less than favorable situation.
There was certainly an emerging sense of Germanness and fed-upness of "foreign invaders" screwing over the country during the 18th century
This is a point you need to be very careful to not mix up some notions, surely there was a sense of Germaness among the princes since the Late Middle Ages, hence why Francis I bid for Emperorship went nowhere, but the concept of Nationalism as people with common history, traditions and/or languages should unite under a single state was something really alien to the Ancien Régime the HRE was tied, in fact the whole notion of Germaness had to do with German liberties and the status quo of the Empire as a patchwork of pretty much independent states, Jakob Moser (pretty much the father of German liberalism) even said "Germany is governed the German way" to describe the situation.
Mind you, Alte Fritz did form a league late in his life to reform the HRE, but said reforms had at heart the objective of conserving the German liberties, this shows how the political theory of Germany was (I don't have Heart of Europe right now so I can't be rich in details about this one).
1) As a starting point, you have to arrest the climb of Prussia. We've discussed this on this board many times, but the Seven Years War is a good time to do it, and you can also go further back. Once the Habsburgs are seen as the only candidate capable of uniting Germany, nationalists will project their ambitions on to them.
A rival is surely troublesome when we are talking about a hegemonic empire, but by the 1760s the central structure of the HRE had already decayed too much, either by Westphalia, French encroachment, Charles VI's reign and the whole WotASS, the Habsburg Empire being the sole German great power may lead it to unified the Little Germanies in the advent of something similar to OTL French Revolution and the Victorian Age, but the HRE itself is too crumbled to salvage.
2) The non-German minorities need to be kept in check. One popular way to do this is to simply cut off Hungary with independence, but this does deprive the monarchy of tax revenues and troops and weaken its standing as a European power. My favoured approach is for Bavaria to be successfully annexed, altering the strategic internal balance. Maybe Hungary revolts and the combined weight of Austria and Bavaria puts them down, and fully breaks up the Hungarian united polity.
Hmm, on a rather detached note, Hungary wasn't part of the HRE, but in personal union with the other Habsburg domains (like Austria, Styria, Bohemia, etc.), Hungary only became a mere unit of a state with Francis I declaration of Empire.
3) Establish a HRE unified military in times of war. This can be done after some egregious Russian or French intervention. Once the principle of all the states contributing troops to the Imperial Army has been added, there can later be ramifications for states not doing their national duty. A sense of national mythology should be established around this proud German army, flying colours etc.
There was already an "Imperial Army" in the form of the Reichsarmee first created by Sigismund and the Kaiserlich Armee which was personal of the Emperor but had recruits and commanders from across the Empire, in fact during the War of the Polish Succession despite Great Britain not taking part of the conflict Hanover still supplied troops to fight the French in the Rhineland. However here comes the trouble: in the case of a standing Reichsarmee, who is the charge? The Emperor? That's a no-oh, the Princes are definitely blocking this as they don't want the Kaiser steamrolling them with their own men. The Diet? Another problem, the Diet was never a truly functional organ and had too much institutional problems, most in the form of really vague laws open to a myriad of interpretations, I mean the Empire had an army, but the Emperor had his own too, it was okay.
5) Accept a nationalist demand for a national Imperial parliament to oversee the Emperor.
Maximilian I already tried it with the Reichsregiment, didn't work out, the Princes tried use it to hold the Emperor to their will and Max didn't like how powerless he was to his own reforms, no consensus happened, but the Reformation did, in the end it failed and was forgotten, I doubt either Joseph, his son or Charles VI's son is going to revive it, specially in the age of absolutism.
6) Establish a central treasury to pay for troops directly, after a war goes badly due to poor co-ordination between states.
Once again: Maximilian tried with the Common Penny (Reichspfennig), it ran in trouble too, first the Swiss refused and BTFO Max, showing the other Princes it was a joke and they didn't have to pay it too, in fact even the Spanish Habsburgs didn't pay it when Rudolph II didn't send help against the Dutch (dude had already enough trouble with the Ottomans).
Mind you, the Princes still paid some stuff, and it did finance (somewhat) wars against France, the Turks during even the 18th century, but once it stopped (the Ottomans made peace and France allied with the Habsburgs) it stopped, the treat was gone, back to the usual stuff with liberties and shit.
Just to make sure I didn't come as a negativist asshole:
maybe the HRE was salvageable in the 18th century (I'm by no mean an authority in the subject, let alone an expert), but I just wanted to point out how the overall structure and ideologies ruling over the Empire prevented (or rather were hindrances) to centralization, mind you the Empire was already "unified" in some sense, there was the Diet, the Chambers of Justice and other institutions, in other note even after two (or three) wars against the Habsburgs the Prussians never left the Empire in the same fashion as the Swiss and the Italians, showing that an identity already existed, but it was not related to a central government.