"At the very most we could expect any Romanisation to get as far east as the Elbe, and even that is unlikely"

The Elbe was undoubtedly what the Romans were aiming for, and confident of achieving, before the Clades.

Some of this is topographical, and 'nuts'n'bolts stuff', Those of you from America and Australia really need to have a map of Western Europe before you when making grand statements here...

Augustus had to stabilise a frontier from the shore of the North Sea which joined up defensibly with the Danube barrier.
He was correct in his analysis that the Rhine would leave a dangerous re-entrant triangle, permanently tempting to Germanic predators.

But look at that map again, and see the border forged from a line of the Elbe running into the Danube - nice and tight, huh?

That is what Augustus saw, and what he sensibly aspired to fortify: a shortened front line is what a great commander is always aiming for (cf the attempt in Britain to push forward to the Antonine Wall)

Even after the Clades, it was plainly Imperial policy to re-establish this "plain as a pikestaff" border. But it tailed away, and by the 2nd century, the Tribes were simply too strong & numerous to incorporate.

PL