... do... do you think pre-Roman peoples didn't practice assimilation?
Yes! That's my whole point. Not literally - you seem caught up on the word conquest too - but the kind of institutional pattern of expansion and assimilation is what was different about Rome
There was war, there were other empires. People intermingled and intermixed, sometimes under rule from another group. There was assimilation, but in an organic process
Rome industrialized the process. They turned it into a mechanical process that has never stopped. It didn't stop when the empire split, it didn't stop when power shifted to the aristocracy of Europe, it didn't stop as America rose as the latest empire after WW2
I do think the HRE was more Rome than Germanic - what language did they speak? Not Greek, Aramaic, or any Germanic language - it was Latin. And in the East you had the Byzantine empire doing the same damn thing, spreading soft influence to Eastern Europe
Christianity became a tool of Rome under Constantine. Jesus said we don't need temples or coin. Jesus was born in the summer. Jews keep the Sabbath on Saturday. Jesus was represented by a fish, and died on the cross so that he could not be used as a tool of control against his people
Sol Invictus was born on December 25, Constantine declared the day of the sun as the day of rest. Sol Invictus is associated with gold. The cross is a symbol of Roman order
Constantine rebranded the Roman religion under Jesus's name, and carefully picked it's practices to control the people
The HRE kept control over the aristocracy through marriage, ceremony, and through relatives in the clergy. They let the kings have their kingdom while controlling the secret little club of European royalty. They held the legitimacy of all of them in Rome.
They also controlled the people directly. They were a parallel power structure. They had a ton of direct power until the 19th century, when things started shifting to mercantilism then capitol
And even now, a few family lines always seem to be the ones in power. The meeting places and the titles change, but each rising and falling empire goes back to Rome