Eventually the upgrade from Android 6 to Android 7 has become available to the user, apparently it took some freeing up of storage space to “unlock” the upgrade.
UPDATE: Afters successfully upgrading (apparently OTA) to Android 7 (Fairphone OS 18.09.2), the updating went on up to Fairphone OS 19.11.2 and from there/then to Fairphone OS 21.05.0. so it’s not a one-stop upgrade from Android 6 to Android 9, but: 6 → 7 → 9. 