I have no FP3+, so I can’t test anything and all I say now is pure guessing: As the Fairphone Android 10 fastbootimage is the very same for both, FP3 and FP3+, there is a fair chance that the FP3+ features the same ro.product.device property, i.e. “FP3” and the difference is in the camera drivers only. In this case, the present builds should work … you may give it a try and tell us about it 
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The GSI LOS17.1 provided here has no original google play services but contains F-Droid, MicroG with spoofing and Aurora Services as privileged apps. On this basis, you can easily install Aurora Store first and than most other needed FOSS and GPS applications as a user.
In case you want to make your own build with even more applications included, feel free to fork buildbot_treble.sh, modify CUSTOM_PACKAGES and make use of it.