You cant because there is a max of 8 secs exposure time and the stock app does not support RAW…
For modification requests you would have to contact Fairphone directly via their support.
Have a look at this App.
The current beta version already has 8 seconds and the developer tries to prolong more if possible. It support RAW pics.
The developer will also try to work on the fact that the camera tends to crash the phone at the moment.