On a sidenote, it’s pretty interesting that Android 9 in the form of Fairphone OS would work with Android 7 encryption.
LineageOS couldn’t do that on the Fairphone 2. While LineageOS 15.1 (Android 8.1) still worked with an Android 7 encrypted data partition, LineageOS 16.0 (Android 9) just didn’t work this way, it could only handle newer encryption.
Before condemning Android 7 encryption too much, keep in mind that TWRP for the Fairphone 2 is able to decrypt Android 7 encryption, but not Android 8/9 encryption (at least not officially or reliably).
Meaning … running Android 9 with an Android 7 encrypted data partition should in theory enable you to do (or keep doing) TWRP backups including the data partition. (I didn’t check that, yet.)