Hey. I don’t think you want to actively use your phone while driving. If you connect your phone to the car, and switch the app the ROVR to start recording, it won’t affect your usage of navigation/map apps, Spotify, etc. So I don’t really think you would need a second phone.
But if you insist, I also have a lesson to share because I had the same thought as you several weeks ago, and I purchased a used Iphone 7 from Ebay. But now it’s sitting in my yard doing nothing, because ROVR app can be only used with IOS 17+ system, which means you would need IPhone 11+ at least, which is at $250+ at least. Good luck.
I have a T-mobile line which I got during a promotion which is FREE. So I have an Samsung S20 FE 5G that I got for about $200. I use it for CRYPTO (until my SEEKER arrives) and for ROVR TX and other things. I have a mount suspended from my overhead near the windscreen and it works great. The battery lasts about 2 hours unless I kill the screen or plug it in.