I was having a similar issue with a 64GB card so I'm not sure if going smaller will help any. I'm using an iflash adapter which seems to be working fine otherwise with the normal firmware. I've tried using disk mode to install rockbox and add music however any time I attempt to boot into rockbox I have issues building the database or even just getting through the menus. It seems there is corruption that happens and then the disk goes read only. Booting back into disk mode confirms this because windows complains theres something wrong with the drive and wants me to scan/fix.
That said, I was able to build the database and play back music using the development build of rockbox from 2020-05-04. It seems like there is some hope. Once I started to put my collection on it and rebuild the database I ran into similar issues again.
I still have more trial and error to go through to see if I can get things working but this is what I've figured out.