That's a bit close, but there is still a wide variety of possibilites.
For instances, with your PoD it is still possible to get rid of the French revolution
and a figure of any significance called Napoleon.

This makes it plausible that
the 'Roman' Empire lives on.
In case you don't want to meddle too much with France in your TL,
or if the Empire does not count as 'united', you need more effective and more
powerful institutions. Although it does not seem simple to solve problems in four
decades which had been neglected for four centuries,
I still see the chance of it. Look at comprehensive reform of 1803:
Something along these lines might have both prolonged the lifetime of the Empire
and enhanced its functions. And by that I do not refer to mediatization and secularizaition,
but to adaption of the stuctures to the
current relation of power (rather than
that of 1400), and codification of "general rules" of the interrelationship of the states.
The reform drowned in the political and military flood originating from France.
In other words, it was just a couple of years too late.
With your PoD in 1780, there's plenty of time to set the right course.
One might argue that the reform only took place due to enormous pressure and threats
from France. This is of course true; but again, there are many options to create
strong, but not as defeating pressure on the German princes to enforce structural improvements.
The 30 years war really wrecked any chance of a unified German state.
No it didn't. True, it wrecked the outlook of the Emperor to gain direct control of Germany
as a whole, as had been the Habsburgs' dream. But a functioning and strong Empire does not
imply that the Emperor is as strong a figure as the kings of Spain or France were.
Of course, such a reform, which even increases the influence of the princes (even though
collectively), is only possible when the Emperor is in an extremely weak position.
(Even weaker than in 1648, relative to his adversaries. In 1806, he was.)