I recommend Peter heather’s work The fall of the Roman Empire : a new history of Rome and the barbarians. He looks beyond some of Gibbons more questionable assertions, such as the Christianisation of the empire being at fault or in fact the barbarisation of the empire being an issue. It also happens to be quite readable, think tom Holland narrative but with a lot more factual and source justification thrown in.

It’s only issue is that it is perhaps not as substantial as gibbons work but I’m sure you could use heather’s sources as a start point. Plus I imagine when reading Gibbon you wouldn’t want something too heavy on the side.