Hey,
You will also need to add support for f2fs in libparted to get it works in d-i (a patch is pending on upstream and in the debian package). Moreover , the crc32 udeb module is needed (required by the f2fs module), it is enabled in the last linux version normally but just be sure that crc-modules udeb is installed. The last days I have tested, only description and meta were missing on some mountoptions (it crashed ramdomly when I was trying to use options). Otherwises, I can format a fresh install and it boots :)
Hope this helps,
Regards,
Romain