Pay securely with an Android smartphone, completely without Google services: This is the plan being developed by the newly founded industry consortium led by the German Volla Systeme GmbH. It is an open-source alternative to Google Play Integrity. This proprietary interface decides on Android smartphones with Google Play services whether banking, government, or wallet apps are allowed to run on a smartphone.
GrapheneOS is the best option that I know of for a phone ROM. LineageOS for MicroG is a decent alternative for de-googled ROMs, and it supports a wider range of devices. It’s not the official LineageOS, but is maintained on a separate site that hosts builds that include MicroG. Official LineageOS does not support MicroG but can be installed without Google crap (and will not work with things that require google play services)