I had to stop using my ipad mini 2nd gen because it was overtaxing it, I am fortunate to have a new ipad mini 5 now. If using your 6plus, I would as a precaution turn off any other apps running. if you begin to see slow down or jitter, it probably isn't the drone. If you have the means, I would recommend a new device.
No-one can say for sure as we don't know what future requirements the DJI software will need, in general although I've seen claims that the software needs decent hardware I've found it works well on some older devices. I don't think there's anything to lose using the iphone for now and then you can reconsider your options if you're having issues with in the future.