Quick cheat sheet I made for myself since I have to do all this (except mkfs) every time I upgrade:
go into advanced mode (luci), software
Update packages button
Filter for f2fs
install f2fs stuff
SSH in
mkdir /mnt/sda1 #or wherever you want
mkfs.f2fs /dev/sda1 #note this formats the disk!
Add line to fstab using vi or whatever:
root@GL-X750:~# cat /etc/fstab
/dev/sda1 /mnt/sda1 f2fs rw,acl,active_logs=6,background_gc=on,user_xattr 0 1
mount /mnt/sda1
Is there a reason you’re using F2FS?
Have you looked into the reports of significant data-corruption issues with F2FS?
I did a quick search to pick an fs with low wear. I definitely am not an expert on it. It would be annoying to lose data but it’s just acting as a media server in my RV so the overall risk is low. It’s not the ONLY copy of Endgame 
Still, I’m happy to hear any recommendations from those who have thought about it longer than I have. Ext4 with journal disabled maybe?
ext4 seems to be pretty robust.
Personally, I wouldn’t disable journalling as routers tend to have the power pulled on them, and it doesn’t seem like an application that demands the absolute, top write speeds possible.
f2fs is mostly for onboard NAND/eMMC for Android - and not really recommended for flash devices that can be easily removed like MicroSD and USB sticks.
As @jeffsf suggests - ext4 is robust for external devices and mount points, and journaling there is a plus when considering the use case - RV, where power might be unpredictable.
x750 isn’t going to really push any USB storage device really hard in any event.