Charlamagne, however, was Frankish and I want this empire to be the WRE, and not a Frankish empire styling itself as Roman.The WRE doesn't necessarily have to survive. Have Charlemagne's empire remain successful, or one of the other German dynasties unite parts of Hispania, parts of southern France and as much of Italy as possible and you have a fairly decent post-Roman rump. Just like otl they'll likely even expand into Germania proper.
Would be hard. Have Odoaecar decide he'll not depose Romulus Augustus, but instead keep him as a puppet emperor and you might have a start. It will be an uphill battle though.