I'll give the Disney Canon credit for at least acknowledging that fundamental hypocrisy, where the EU pretty much glossed over the fact that the Outer Rim pretty much went from being powerless and exploited under the Old Republic, to being better in some places and worse in others under the Empire, to being powerless and exploited and told to be happy about it under the New Republic.
I don't think this is accurate. The Separatist grievance, at the end of the day, boiled down to "the Core has all the power, only pushes policies that benefit the Core, and the Rim gets exploited without ever having a say". Fundamentally a sort of "no taxation without representation" kind of deal. (We can argue at length about the megacorp hypocrisy and astroturfing, but I'm describing the real popular discontent the CIS tapped into.)
The Empire crushes the CIS and was basically Core supremacy on steroids. (Tying into the human suspremacy that was linked to it, et cetera.)
The Rebel Alliance was, at least in part, heir to the CIS grievances. They were primarily Rim-based, multi-species, and overtly anti-centralist.
The New Republic was basically what the anecedents of the CIS argued for, before they ultimately saw secession as the only way out: an extremely decentralised confederation of worlds/systems/whatever. With as one of its key provisions a
unilateral right to secession. So while the same Core elites ended up in charge of the Core, their ability to exploit the Rim was mostly gone. If they did that again, any Rim system could legally secede without any fuss. (Indeed, during the fake Thrawn debacle, various systems actively did so.)
The New Republic didn't make the Rim rich, and didn't invest in the Rim in the way a sort of... space-Henry Clay might desire. But that's because the Rim itself was demonstrably anti-centralist and followed a "we just want to be left alone" line. And that's what the New Republic gave them.
