Use a rooted Android phone with AFWall+ installed, with default block rules. Even just LineageOS allows you to set granular network settings per app, though it's not preemptive like AFWall.
They're the ones allowing you to root your phone or flash a custom ROM in the first place, so that's not a fair characterisation. Banks have a vested interest in reducing fraud, and a rooted Android might allow for easier and additional attack vectors into their apps and thus systems.
Aaaaand no.
Even in the case where I’m willing to risk trusting the developer, they have literally zero resources to pen test the software I’ll be running my banking apps on, and in the case of Android roms need to run known vulnerable software (out-of-support source-unavailable binary blobs for proprietary hardware that were never open-sourced).
The same argument was made about TPM’s on PC’s and against Windows 11 for years (that they should just be disabled/sidestepped). It only holds water if you don’t understand the problem the device solves for or have a suitable alternative.