Here the simpeline

    #!/system/bin/sh
    REALMNT="/mnt/media_rw/extSdCard"
    mount -t ext4 -o rw,noatime /dev/block/mmcblk1p1     "$REALMNT"
    sdcard "$REALMNT" /storage/extSdCard 1023 1023 &

And here writable ext4 on external sd

    #!/system/bin/sh
    chown media_rw:media_rw /mnt/media_rw/extSdCard
    chmod g+w /mnt/media_rw/extSdCard

You can change "extSdCard" like "sdcard1" where your external sdcard mounted

Iam used stockrom with Kingroot, but I try Supersu its trouble with mounting filesystem