Two things to consider as well:
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I don’t know if you have checked, but the update to Fairphone (Open) OS 19.11.2 removed TWRP from the FP2 (and replaced it with the stock AOSP recovery). So if you want to use TWRP, you need to re-install TWRP manually (but maybe you know about this and have already done this).
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Fairphone has been working on an upgrade to Android 9 for the FP2 for quite a while. This work is now in a stadium that makes Fairphone foreshadow “more news” on this “very soon”. So you might want to consider if it makes sense for you to wait a little longer and make both steps in one go – going back to Fairphone OS and upgrading to Android 9.