Unable to install GmsCore properly with Magisk and A/B system

Device configuration

Device: Xiaomi Mi A1
ROM: AOSPExtended
Android version: 8.1.0

Magisk version: 20.4

Description

I installed microG Services Core as well as microG Services Proxy. microG is fully set up (all checkboxes checked).
However things do not work as expected, since even after signing in to a Google account an app requiring to verify a license fails, and Maps crash everytime.

Installation steps

My installation steps were rather unusual due to my device (equipped with A/B partitions) and Magisk's design.

Prerequisites

1. Enable Signature spoofing
   It was supposed to work out of the box with my ROM, but installed the XPosed module for good measure, since I couldn't find where to enable the option with my version.
2. Get rid of some packages
   This step took me way too much time. Removing the packages from /system/priv-app wasn't easy because of a lot of problems (unable to remount system to rw, thought dm-verity was interfering but it seems it didn't, successfully removing packages don't last after the reboot...). Had to install the Magisk module debloater to solve the issue.

Installing

3. Install GmsCore.apk
   Same problem than previous step concerning /system/priv-app. I pushed it there as advised and the app didn't appear anywhere, and after a reboot it was removed. Solved the issue by installing GmsCore.apk as a user app then used the Magisk module systemizer to make it a "privileged system app".
4. Install GsfProxy.apk
   This is the only thing that didn't bring any trouble.
5. Reboot device
   I already rebooted my device an unholy amount of time because of the previous steps, did it after installing GmsCore (twice or thrice with different methods, only talked about the working one).
6. Disable battery optimization
   Battery optimization was disabled only after the "systemization" automatically.

What could be wrong, and how should I fix this?

The report names Xiaomi Mi A1, AOSPExtended, Android 8.1, Magisk 20.4, GmsCore.apk, GsfProxy.apk, and the debloater and systemizer modules. Start by reproducing the stated installation sequence on an A/B device and checking package presence and Maps and license behavior after reboot; done means a supported installation remains present and the reported failures no longer occur.