Mmh, thanks for trying, but that didn’t really shed light on the thing for me.
That is, or at least wasn’t, true, as you can see here: https://community.e.foundation/t/ota-upgrade-for-fairphone-3-not-available/46252/7?u=sebastian.
The core question remains, if there is any way to update 1.17 S dev without data loss. ADB sideload would be the way of the walk and has worked in the past, but it does not with the official 1.18 S stable image.
Any ideas anyone? Or do I have to cope with installing everything from scratch?