mm, then i would:
1. factory image (clean base)
2. root & TWRP
3. flashed Chroma Rom, Gapps and CodeBlue
3.a boot rom, configure first boot setup
3.b reboot recovery
3.c wipe caches
4. nadroid backup (/data only)
5. copy backup to PC, renamed "ext4" in data-backup name to f2fs (also the md5 files)
6. converted data/cache to f2fs in recovery, reboot recovery
6.a check it is still F2FS
6.b flash codeblue again
7. copy backup files back to the phone, restore data
7.a check it is still F2FS
7.b reboot recovery and re-check /data and /cache are still F2FS
8. reboot the system
9. after setup of Google account etc. I rebooted to recovery again, and now cache and data were labelled as ext4 in the advanced wipe section.
If that still fails, either your rom, kernel, of the gods, wont do F2FS
Thank you for your help and quick answers! I will try that, should I use a specific version of TWRP? like 2.8.5.2 (.3 doesn´t allow MTP) or 2.8.6.0/.1 or some other version? Some seem to be buggy/not correctly working. I just want to avoid another hour of f&f (flash and frustrate)