The phone app works well under certain conditions. I've used it professionally to both film and take stills of events I've been at, but universally it was under 'easy to use' conditions - i.e. when I could use both hands walking around. That's not going to be the case on a bike. A lot of 'virtual buttons' (on a phone screen for example) become unresponsive if you're wearing gloves or your finger's wobbling or either your finger or the screen is wet - all common bike scenarios. I ran a 1/2 marathon last March where it rained on the start line and it took a good five minutes to get the start button on my Apple watch to respond.

I've got a GoPro remote that has actual buttons to press to start and stop the camera and that works well but again, using it on a bike is awkward. I filmed this sequence a few years ago with the remote hanging round my neck on a lanyard -

https://www.youtube.com/watch?v=1nFf6fKenaM

Actually finding the remote as the wind moved it around was the hard part. If it had been in a fixed position it would have been easier but that's with the advantage of hindsight. Giving it to the pillion (if you have one) is better but you lose a bit of 'the director's vision'. When we were touring Canada a few years ago I tried that approach but we ended up with some very odd bits of film where the stop start instructions got lost between rider and pillion.

Some of the newer GoPro's have voice control (mine are all old ones) but I don't suppose that works well on a bike with all the wind / ambient noise.