What’s the downside of lowering the required insanity to enter voidform? It solves ramping complaints without much issue. With the intent that we will be in voidform more often, it can be balanced around that idea. It also doesn’t completely wipe away the building/draining playstyle that some enjoy.
As others have suggested, final talent row can remain mostly the same. Dark Ascension can become a flat damage increase on your next void form, aka a burst cd. Surrender to Madness can either reduce insanity decay or provide a static insanity supply to allow more movement. Now you have a faster playstyle, burst cd, movement talent as our final row… As long as the modified talents equate to what legacy of the void would provide, it would be balanced and more so about preferred playstyle
Lingering insanity would need to remove the haste when entering your next voidform (as it’s purpose was to improve downtime between voidform, not buff the subsequent one), and now that talent row can see more action too.