mounting an app folder to external storage fails for Write access on Android Q but will work in Android Pie

I could mount my partitions holding whatsapp (exFat) and titanium ( ext4 ) backup data on android Pie but right now on Q it will not let a RW operation be performed after mounting I understand /sdcard is a virtual emulated FS and the fact that there are app specific views etc. But after going through all that perplexing stuff, I couldn't crystallize on a working solution. I've gone through a related Q on stack but none of these approaches work for me.
So lemme explain a bit where I come from
: {} I can successfully bind mount /storage/emulated/0/WhatsApp to exFat /storage/[UUID]/WhatsApp for RW. Obviously because exFat is Permission unaware file system. Did a regular mount and it will mount to /mnt/media/runtime/[Read & Write & Full & Default] with 3 separate 'mask' options mask==6 , 7 and 23 ( BTW that mask option isn't quite your umask . Its '*decimalized*' umask -convert that 7 to binary to get real umask ). So in fact it invokes the same path as mentioned in the article behind the covers viz /mnt/media/runtime/[Read & Write & Full & Default]
{} I need to similarly mount an ext4 partition with Titanium backup. Here are the issues. If I mount /storage/emulated/0/Titanium to ext4 /storage/[UUID]/Titanium ( like I mentioned in fact it mounts to /mnt/media/runtime/[Read & Write & Full & Default] with 3 separate 'mask' options underneath. With this mount. I can Restore Apps but cant backup Apps. So R is allowed RW is not and I granted Titanium Root access. Maybe the explorer components of that apps are not using Root level access. Somewhere i read - mount using correct options of sdcardfs and 'fuse`. My question is *precisely How* ?
{} I found somewhere a bindfs binary but it does not work with my CPU Architecture
{} Now with everything the same I Could RW using above approach in Android Pie . This is an issue with Android Q ...what changed ? how do I work around ?