This depends on what you're trying to defend against. In my opinion (on GrapheneOS):
"Accessibility" permission (i.e. full control of the device)
"Network" permission
"Modify system settings" permission
"Install unknown apps" permission
Any permission that allows apps to communicate with one another (such as a reduced sandbox, file permission, or app communication scopes)
Those are the only permissions that I can think of off the top of my head that could potentially allow an app to phone home. Turning off Wi-Fi for the device does little if the app also has the "Wi-Fi control" permission.
At one time, the bank that i used decided to made an app, and they demand Location, Camera, Contact, Files, Microphone, and SMS, in which they will ask for all of it from the get go and not allowing either one of it will send you in a loop, unable to use the app at all. I bail immediately and continue to use the website.
As for the scariest one, camera. They can see where you are and what your surrounding like if they demand "always allow".
This stuff makes me grateful that my bank and your bank still maintain a fully-featured website. I would be quite upset if I were stuck with such an app and no website.
After the incident they did made some change to only ask permission for the appropriate function and can allow "while in use", and gotten rid of location permission altogether, but that incident kinda open my eye on cybersecurity and privacy, because if bank can hire subpar dev for such an important app, then all those gadget with IoT will not have top-notched dev doing their app. I'd rather be a luddite than lose anything important.