I am not a hacker and I don't know anything about Apple products.

But there are a few weak points on your iPhone:

-Apple has synchronization stuff.
-Your personal phone is online100% of the time and your seed was not even encrypted.
-You may have been exposed to phishing sites on WhatsApp/telegram groups, emails (I know that's unlikely,but your personal phone is more exposed to that than other devices).

Storing seed on a piece of paper is 100% anti hack, unless he has physical access to the paper.

Think like this.

Every time you write your seed (or scan photo etc) on a device with internet connection it is considered exposed and you should transfer your funds immediately. Your seed was online 100% of the time (not matter that it was locally on device, because the device was online).

It's very sad what happened to you. I would be devastated. But don't let this move you away from this technology. Use this opportunity to learn. You will make that money again.