Thanks for your advice. We killed them this week. We now let tanks and healers jump together, too. But we start to coordinate the jump as around 70 (don't knos why, RL insisted). The actual jump often was around 80. We did not tank the adds anymore. However I think it would be faster and easier to heal if we would tank them, again. When we tanked them they were easily dead around 65 and now sometimes one or two flowers survive. DPS changes for ultimate when flowers are dead or at the last moment.
Each group probably has to find their own strategy. As soon as you know how to coordinate the jumps it's only about balancing your dmg right. How you do this depends on your setup and how much every player is able to contribute.