usermod -aG video,input dietpi is required at least. But best in this case is to use the LightDM login mask and the login with any custom user. If I’m not wrong, LightDM can autologin internally as well. Best is actually to create an own login user and grant it the required permissions for daily usage.
It’s somewhere on the ToDo do actually remove the dietpi user (only leave the system group for shared service access) and instead have an option to create an own custom login user with selectable permissions on first (root) login.