sd card - mounting multiple ext4 partitions in Android 7+

I am doing away with exFat for my Microsd card after all my pictures decayed , corrupted and the directory was locked. I cant seem to write into a specific directory. chkdsk will just plain abort saying :

The disk does not have enough space to replace bad clusters

detected in file \WhatsApp\.Shared of name 1.
Corruption was found while examining files in directory \WhatsApp\ (6).

so I 'd like give a shot to Ext4 partitioning that cheap micro sd card , not just 2 ( the standard app2sd and sdcard1 design ) but 3. One for apps and other 2 depending on the write activity , I tweaked the block parameters etc. for each
I am able to mount the partitions & a rooted Total Commander can see them all BUT other apps cannot see the partitions I created.As I understand vold helps other apps be aware of this. So How do I go about getting this 'formally' with sounding 'hackish'
what I did :

mkdir /storage/
mount -t ext4 -o noatime,rw /dev/block/mmcblk(x)p(y) /storage/
where x and y are variables from partitions file in the proc dir.

I read about a fstab. file out there that can be modified but it would be overwritten every boot.

even the 2nd partition that app2sd ( or link2sd) creates can ONLY be approached as root. So prolly those apps that ask for root can see this stuff but other apps can write to it
Ext4 is linux . Sounds like Android seems to ignore its own 'biological' kid ext4 and instead recognize a stepson 'exfat / FAT '
I am sure I am missing something here. Any ideas ?
I am rooted with Magisk on a Galaxy j7 with of course TWRP ( 3.x) installed

Print env o/p