Completely powering down is just that. That includes your camera not being able to run at all.
As far as running a separate device, I can understand why you would need a different device, but for strictly managing finances? I manage finances just fine on my daily driver. I sincerely hope you aren't doing anything underhanded here. Most people get a second device for backup (spare device), work, or development/rooting purposes.
At least on my new phone (Moto g6), it doesn't boot completely until the passcode is entered. I am considering getting a second phone as a spare soon, but not a Moto g6. I've always managed that... especially important for me, as it's been years since I've had a landline at home.