Welcome to the community forum.
The closest to the Google-certified OS by Fairphone should be the combination of the following:
-
LineageOS … https://wiki.lineageos.org/devices/#fairphone
A community supported Android OS based on AOSP (the Open Source base of all Android OSes) -
MindTheGapps … https://mindthegapps.com/
Those popular non-Open Source Google Apps and background services as a convenient package to install on Android OSes not having them (and not having microG as a replacement, there can only be one or the other) -
Google’s registration of devices running uncertified Android OSes for the legitimate use of Google services at https://www.google.com/android/uncertified to play it safe with your Google account.
(If Google still continued this. I haven’t checked for ages and can’t check right now.)
Based on priorities and perhaps curiosity, other options might be worth checking out, of course.