Do you NEED a dedicated device for the Mavic? NO. But it's definitely optimal. Your iPhone 7 Plus will work just fine until the day you try using a iPad Mini4 or similar, (or larger), then you'll never go back to using a phone.
If you've already spent $1000+ on the mavic and accessories, I think it's definitely worth it to spend another $200 and get the iPad Mini4 which will make the experience at least 25% more enjoyable.
I think the iPad Mini4 is the favorite/most popular because:
a) the IOS system is much more stable with the mavic
b) it fits perfectly in the RC arms without having to use any type of mounting bracket.
c) it's small enough to have very little affect on the portability of your total mavic setup.
d) they're readily available for a reasonable price, and a proven durable, reliable piece of hardware.
I've used the iPad Mini4 for 9 months now with zero problems. 16GB version, WIFI only. Never have I missed the GPS capability or needed more GB space. You can probably get an iPad Mini4 on Ebay now for around $200 new or $100 refurbished.
I had to use my phone once when I forgot to bring my iPad and it felt like watching a movie on a 13" TV - horrible.
I got the
DJI Goggles a few months ago, so now I use them about 75% of the time.