Booting Fairphone Open OS automatically after sideloading from TWRP
Road to answer
Because I did not clearly understand the difference between selecting, Wipe&gt;Advanced Wipe&gt;Data&gt;yes, and Wipe&gt;format data&gt;yes I did not thoroughly/consistently test the latter option as I thought it was the same as the Wipe&gt;Advanced Wipe&gt;Data&gt;yes option.
Since I experimentally determined that a factory reset did fix the issue, I focused on automating that option in the given constraints. Doing so in this question, I was reminded of the Wipe&gt;format data&gt;yes option again and provided with the adb command in a comment. Since I did not yet find a factory reset method I tried the sudo adb shell twrp format data command manually, and it worked. Then I tried it again in the automated setting with: format_data=$(adb shell twrp format data) and it still worked. This answer has been made possible thanks to the comments of alecxs.
Answer
I was able to automate the installation and booting of Fairphone Open OS using the following set of commands:
sudo adb shell twrp sideload
sudo adb sideload 19.11.2-sibon-24df0be9-ota.zip
sudo adb shell twrp format data
adb reboot

That made the phone successfully boot into the new instance of Fairphone Open OS.
Note
The termination of the sideload modus was not needed anymore, hence the following lines were ommitted:
# terminate/end the sideloading modus
sudo adb sideload /dev/null