If there is a community solution out there, it’ll likely surface here:
Judging by the info there, and info in TWRP’s github, decryption requires some vendor-specific stuff to be built into the image, possibly together with some ROM-dependent stuff.
One comment there is:
There’s not necessarily a “right direction” that anyone can lead you. On some recent devices, you have to hvae qseecomd running in recovery. On some devices you have to mount another partition like the modem or firmware partition. It’s easier when you have full source for the ROM (Nexus or Cyanogen Inc devices) because you can compile an engineering boot with full adb root and grab logcats and run test binaries during a normal boot and use a process of elimination to help identify what needs to be done.
I think the Fairphone developers are busy working on the Android upgrade, so don’t expect it to be solved by them before that lands (as probably the new android version would break decryption from recovery again anyway).