No. Immense is as big as it gets. However, I've found one way to slow down things is to mod in a range penalty. When you combine it with the right settings for stars and planets, you can have what feels like an even bigger galaxy than normal (and Immense is still pretty big).

I think that would mean Rare for stars and planets, as opposed to Abundant.  With Tight Clusters, too.

This game is far too buggy, even for immense. Eventually it reaches a point where auto-save causes a lot of crashes. Althernately, if you have a custom map with a lot of planets, eventually the bogs down to the point it takes 45mins or longer just to process a single turn. Either way, super-huge maps, which is what I prefer, are not quite impossible to play, You have to learn to tolerate constant restarts to get through a decent long, immense map. SD has shown little inclination to patch or fix this very buggy game. Sure, its fine if you keep your maps small, but if you like em big, its an exercise in frustration. The game simply cant deliver the goods.