There is an overlay indeed:
G_CONFIG_INJECT 'overlays=' 'overlays=gpu' /boot/dietpiEnv.txt
reboot
With GPU you mean hardware acceleration? HDMI works OOTB, doesn’t it?
However, important enough to add a dietpi-config option, indeed. Would be still nice to see which features exactly this enables.
You mean when you run poweroff, it indeed does a full reboot cycle?