The garden which I moved to some years ago has a gigantic rose which I was led to believe is 'Kiftsgate'. Given its height - about 35ft - and its hip formation I have never questioned its identity. But, reading your description, I have come to doubt this since each of the flowers making up the cluster has 10-12 petals.

I am in USDA Zone 6b in central Kentucky USA. I planted Kiftsgate in 2016 in a wind-protected location adjacent to a Black Locust tree. The first year it mounded up. The second year it grew over 20 feet up into the tree but then died back to the roots during the more-or-less normal (whatever that is now) winter of 2017-18. I removed the 20-feet of dead cane and the rose resprouted but the new growth and roots were then totally killed by one of the series of late frosts that we had last Spring. Although I tried to protect it, I live in a windy location and the winds high up in the tree may have been the most important limiting factor. Please see photo of one-year cane growth attached to the side of a building.