I would suggest Final Cut Pro X. I don’t know your budget but don’t know your skills either (if you are a beginner or an enthusiast). But it’s a good move to future proof your skills and learn on a pro app (that’s not that expensive anw) as you don’t know where your road will take you.
As for filming pro, I use it for shots I can’t get with my Sony fs7II, mostly macro shots or incognito situations, and the quality is just great from my 6S. Hell I even prefer it from a GoPro hero 6 black mostly due to the focus ability of the iPhone. I think there is a log setting in the app that’s a premium feature but I’ve seen examples and it’s a nifty creative tool.