Not got an iPhone. But with an external battery this would be an easy option.
Instinct, just start it in the morning and stop it in the evening. Then wait 5 minutes while it slooooooooooowly saves the activity. Not sure why you think the buttons are “tiny”, tbh.
Edge 830 probably won’t have enough battery for a full day’s activities. My 530 can manage about 7 hours or so, granted that’s with Varia + HR + power connected – assuming you turn off all unneeded externals you should get a bit more.
I’ve got an Instinct, and my instinct (ha ha) tells me to go with that as it’s easy.