How to use an ext4 formatted sd card as external storage with working app permissions?
I simplified my way to apps see my ext4 formatted [1] on internal something apps will see folder usb1 and may read it (I have to check the write) .

[1]

mkdir /data/media/0/usb1
su -mm
mount -t ext4 /dev/block/sda1 /data/media/0/usb1/
mount -t ext4 /dev/block/sda1 /storage/emulated/0/usb1/


Full answer here : https://android.stackexchange.com/a/204616/119381