u can leave lahore early in the morning like 4-5. try to avoid driving during the evening especially in interior sindh.
if u leave by the above time, you can reach sukkur by 2-4 in the afternoon.
after that u can be in karachi between 8-10 at night.
Sind is safe after crossing sukkur. CNG is widely available. but try to speed up the travel by using petrol also. for eg. i will not recommend to waste your time filling up CNG every 150km.
the above calculation is based on average speed of 100-120km.h