I purchased a Delta Cycle Smart Cell phone holder at my local bike shop and couldn't be happier (I can't post links, but it is also available at Amazon, just type in those words). It holds my phone very securely with just the bungee cords, but for rough trails, there is also a flip switch on the back to lock it into place, and it's impossible to take out without unlocking. However, the phone doesn't need a special case or anything to mount to the holder, so taking the phone off the bike takes only a second or two. I also love the optional stem cap mount, so it is rock solid and doesn't spin around my handlebar like the last phone mount I tried. I use it with a Samsung S8+ and just the built in Samsung health app, and it does 90% of what I want without buying anything else (no cadence counting, though). It also has a little bump out thing on the bottom that lines up with the speaker of my phone and acts as kind of a megaphone. It's a great inexpensive solution, probably the best bang-for-the-bucks I have spent so far.