It's better to use a device that has never seen the internet, I think an iPhone is a security hazard in all situations.

You can burn a live CD with Tails OS for example and run it on a disconnected PC, the electrum wallet can generate the seed safely and you can then store offline, once rebooted the PC the data are gone.

People like @DarthCoin gave me lots of advice and will surely be much more knowledgeable than me.