I frequently travel on N5 Karachi - Lahore - Karachi. About this route i can recommend that you leave Karachi early in the morning say two hours before sunrise. That way you cross sind in day time.
Once in punjab, road and traffic conditions are relatively much better. If you guys are comfortable sharing driving seat (more h than one driver) you can easily reach Lahore in a stretch. Otherwise Rahimyarkhan or bahawalpur are options for night stay from where you can continue next morning.
There is another route from multan where you don't have to go via Lahore but i haven't used it personally so can't vouch for it. Heard it saves you 4 hours drive
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