Ball Valve Seat Damage

I would inquire of the manufacturer as to what the allowable differential is. And also study the system to see whether a small bypass valve actually can equalize the pressures (or nearly so).

If it's an isolation valve, then it won't get operated very often. This factor needs to be taken into account when selecting hardware and piping.

Can a existing similar system elsewhere be copied?

as someone replied, you should consult to the vendor.

and, the type of the ball valve will make the things differently.

for this application, trunnion ball valve is much better than floating ball valve.

I have seen ball valves get damaged from opening under significant differential pressure. There are different styles as has been indicated. The pressure imbalance presses the edge of the orifice in the ball into the seal and cuts it. This is in industrial type hydraulic valves. I have little experirence with valves above 3". There are designs that have better bearings and balancing of the loads that last longer and don't suffer from this they are rated for more openings prior to failure.

I think your idea of using a smaller valve to decompress and thus equalize pressure is a good one as long as the high pressure side is not continuosly supplied with fluid that will saturate the smaller valve so the pressure on both sides of the valve can equalize. This will maximise the life of your larger more expensive valve and will be easier to control the rate of decompression potentially eliminating the decompression shock that could result if the large valve is opened rapidly.