Hi and welcome to the forum
Your situation is all too common. When you select ‘internal’ storage the contents are encrypted along with the rest of the phone’s memory. As you have discovered when things go wrong you may loose a lot of data.
There is no publicly available option for the common user to revert this via the phone, it often requires a paid for solution.
Try starting the phone in safe mode, this will disable any custom apps and settings but not delete ‘data’ ~ It’s a long shot.
https://support.fairphone.com/hc/en-us/articles/360048843571
You can remove the card and insert another and set the new one to ‘external’. See if you can access anything on the phone and transfer to the SD to ensure it is working.
Replace the original SD and see if you can read anything, without any new formatting. There’s a slim chance of access and then you can try and copy to a PC before you reformat the SD
See