AOA sir jee,
You don't need hard tyres in Mehran, because its suspension is already very stiff. So Hard Tyres + Hard Suspension results in rough driving experience. Get some softer tyres, for this purpose Yokohama is best choice. Also tyre size matters are alot, means if you put even little bigger tyres then it will definitely result in bumpy ride and eventually effect you suspension with passage of time.
Your cousin's 2006 Mehran might have original size tyres and might be softer than yours. In my Mehran I had Maxxis tyres but of same size of General tyres and the ride was smooth.
So get softer tyres with original recommended size by company. For test purpose you may also exchange tyres with your cousin's Mehran and see if it makes any difference.
Happy driving 