finally got it working, and GCam doesn’t complain anymore! thanks @Aaanze + @Error for the tips
for anyone interested, I’ve finally created an androidfilehost dev account and uploaded my build with microg preinstalled here: lineage-18.1-20220206-UNOFFICIAL-FP4.zip | by g4b for Fairphone 4 (md5 8d810026d4a470eaefc56e544f014dc3)