I made a Mi9 kernel with F2FS support and other stuff (Fsync off, GPU OC 830, CRC off if wanted). Yes it has F2FS encryption enabled in the build. Looking at what you do here I would need to use grep to mount the partition? I can format in TWRP to F2FS but TWRP can't mount it. Looks like you are bypassing TWRP with grep via Linux? Trying to figure out how to mount this and run F2FS you had luck on Poco F1 Mi9 same brand... Also /vendor/etc/fstab..qcom can be edited in a root browser and the changes stick.