Except, again looking at Star Wars/A New Hope, they mention an Old Republic. What happened to that Old Republic? Why is there a senate to be dissolved if this Republic has been gone for so long? Is the Empire new? How’s it gain so much power so quickly?

Snoke doesn’t undo anything of the original trilogy. Palpatine not being defeated does, but a new threat coming from the Unknown Regions does not.

It’s not any more or less established in universe, your need for every answer coming from a 30 year gap is not the same as a flaw in storytelling.