emperorren:

The opening crawl is everything they need to cover that period in not so many words, regardless how long it is.

As for what *happens* offscreen: Kylo consolidates his role as Supreme Leader, while Hux consolidates his role as Kylo’s antagonist. The First Order is superficially thriving (like the Empire in ANH), but is plagued by simmering tension and intestine conflict. They might be ruling a solar system or two. Possibly a good part of the galaxy. Someone is building up a rebellion somewhere. The Resistance is either holed up somewhere and struggling, or has rebuilt a base somewhere and is gaining new followers. Rey has grown into her role as the last Jedi, and Finn in his role as one of the leaders of the Resistance. Rey is slowly making sense of the Jedi texts (and might learn something that will become relevant during the course of IX) and is on relatively good, friendly terms with everyone in the Resistance, including Rose, who has recovered from her coma but hasn’t dealt with her feelings for Finn yet, and Poe, with whom Rey has some latent conflict that will explode later on in IX. Leia is (probably) alive and in charge, though something will happen to her when the time jump is over. By the end of the time jump, some catalyst happens, making the FO and the Resistance (and possibly Kylo and Rey themselves)

clash

directly again. The force bond has seemingly disappeared, only to flare open again at the beginning of episode IX.