further playing around, I think there would be space on the sound & pattern screens to add a title and tab select control?
https://learn.microsoft.com/en-us/windows/win32/controls/tab-controls
This could let you easily switch between the 2 screens for a given voice. Also the mechanic of moving the DPAD from the sound & pattern center areas out to the bottom controls for BPM & pattern select might be able to be accomplished just with the dpad? similar to if you were to code the tab control, just press up an extra time to get up there.