I agree with everyone else who says to use a dedicated phone for the bike computer. No need to get the latest iPhone for $1000.

A used phone that's a few Android versions out of date will work perfectly for this task.

My bike phone runs Android 5.1. (Android's worst version) and it's fine for this purpose.

The advantages of having a dedicated bike computer are

• You set it up as a bike computer so it starts all the relevant apps when it turns on. Mine loads the speedomete/odometer app in the foreground, an MP3 player in the background, a weather temp widget on the status bar, and an app that turns the volume keys into song advance buttoms (to control the music without actually going to the MP3 player app) I also have different settings on my bike phone than my real phone. My real phone turns off the screen after 15 seconds. That's not handy on the bike. The bike phone is set to full brightness and stays on unless I tap the power key to turn the screen off.
  
• Speaking of special settings, do you use a lock screen on your regular phone? That's REALLY annoying on a bike computer. You don't want to be trying to meddle with a key code while you're riding. The bike phone can have the lock screen disabled.
  
• I don't have to run all that stuff on my regular cell phone. It just automatically loads on boot.
  
• I have it loaded with maps of everywhere I might possible end up on the bike.
  
• It's replaceable. So if it falls off the bike in the street and gets run over by a semi truck I'm out $50, not $400. Meanwhile my actual phone is safely nestled in a secure bag.

Things to look for in a phone...

• The screen has to be about inches or better or it's hard to see at a glance while you're riding. You don't want to be looking closely at the screen while you're riding. You want to glance at it and see what you need.
  
• Cheap. It's riding in the open air above a hard asphalt road. If it falls, it's probably done. Make sure it's replaceable.
  
• Front facing speakers. This is nice. Not a must have, but really nice if you want to use it for music. Newer phones with front facing speakers can be LOUD with excellent sound quality. Totally eliminates the need for a BT speaker if you're into that sort of thing.

Things it doesn't need to be...

• Waterproof. This is a 'probably' situation. If you are a fair weather rider you don't need a waterproof phone. Yes, a rainstorm might sneak up on you once in a while. For the rare times that happens, take the phone off the handlebars and put it in in a pannier or something. Have a zip lock baggy with you at all times to put the phone in. (Good idea for your regular phone too). If it is waterproof, great. But if you're in a pouring rain how much do you really need to be staring at the bike computer? It'll still calculate your route and distance from inside a ziplock bag inside a pannier even if you're not watching it.
  
• Great battery life. Yes, great battery life is a good thing. If you can find one with great battery life, score! But if you can't, just turn the screen off when you're not looking at it. Tap the power button to turn the screen on when you want to check your millage for 1 second. Also with a dedicated bike phone you can put it in airplane more (GPS will still work) and tweak the settings to make it battery friendly. Even with the screen on constantly my bike phone lasts me 4-5 hours. If I toggle the screen off when not looking at it, it'll last me a week in airplane more running 24/7.