You can also flash an update. You don't need to decrypt to flash it.
If you want to back up the encrypted data partition you can do it in rom. You can use dd if of command and keep the backup to your PC.
After doing a format and flash the system, you can restore the backup same way in TWRP, since after format TWRP can access data before first boot.
F2fs is useless, but it's default.
Mtp works fine. Only adb sideload bugs a bit.