using Graphene OS is something I do only when I need to add pictures or do a quick withdraw for example. It depends on your setup, phone network or internet only using phone with no service. Still using a phone for doing constant business is not recomended. An old laptop and an OS like tails which boots from the usb drive and does not store data outside of the persistant storage on the drive which is protected by a password that if lost or forgotten cannot be recovered even by yourself. There is an entire article, more of a book that outlines absolutely everything for saftey and security that you can find on dred, I have seen it on Torzon, and I am sure it is here somewhere as well. If you really want to use a phone then you might want to spend some $ and get a phone called librem 5 by purism, but then you are spending alot. You can get an old laptop and download tails os for free and save alot of money. Securing your wifi network is also a problem that should be considered. With phones data is always being collected by this app or that app, the service provider itself does this. and this leaves trails, bits of info. Nothing is 100% secure as far as I understand though.