I’ll just chime in and say two things.
Firtsly, there shouldn’t be any reason for the data partition to be corrupted. The A/B partition layout includes two system partitions, let’s say system_a and system_b. One of them is active, the other one is not. When installing an update, it is installed into the inactive slot. When the device reboots, the slots are “swapped”. The data partition is never touched.
Secondly, Fairphone (and most other OEMs) always warn you, that the device may reboot multiple times before the update is installed. The required 7 reboots shouldn’t take more than 15-20 minutes to happen on their own. Applying patience and just letting the device do its thing is the best thing to do.
Agreed, maybe the FAQ on Fairphone’s website should mention the mechanism, but it was also specifically designed in a way that doesn’t require any user interaction (“fool-proof”, if I may), so I understand why its not there.