I would try this first. I had a similar issue with one of my 128GB MicroSD cards dying not long ago. I backed up the entire MicroSD to my PC, formatted the new one, copied everything over to the new one, and once it was working then updated to the newest firmware for everything. Still going strong.If its a corrupted sd,there is a simple fix
Backup all the files to the pc
Then format the sd to fat32
Once thats done,download the latest version of luma 3ds and put the boot.firm on the root along with a folder called luma. If done correctly you shouldn't have a problem booting unless you corrupted your nand.
My MicroSD card did not die to sleep mode but I did find that there is some sort of battery drain going on. After a while it seems to drain the battery even while the system is off. I never did find out the cause, I just charge it when I want to play it after the battery dies. I hope you get your fixed.