Just a suggestion, really. I have a tendency to read original works first before I find shortcut versions of them, just to supplement a bit more. Nothing beats originality though and the type of language and story from an era that you will not find in newer versions.I'm sure you're right in general, but I don't think it's possible to summarise Gibbon. Half of the charm is the way he uses language. although it's very complicated and 18th century language lol.
If we want to know about that era in history there are many more modern and accessible books to be honest