The goal is to only schedule vt_timer when the screen needs a refresh.

Do you see missing places where the timer should be restarted?

I use the following DTrace script to see when the timer runs:

#!/usr/sbin/dtrace -s

#pragma D option quiet
#pragma D option bufsize=16m
#pragma D option dynvarsize=16m

fbt:kernel:vt_flush:entry
{
  printf(".");
}