I did see a recommendation elsewhere on the web to place a short jumper wire between the two ground pins, and the shorter the better. Because I have not seen the schematic of the DFplayer board, I can’t say for sure why they even have two ground pins, but it does make sense to connect them together as well. The implication is that the board is only a two layer board, which means it probably does not have a good ground plane, and this can cause electrical noise issues. Providing a short jumper between the two pins might help a bit. Aside from this, it seems like you have the hardware side covered, assuming the DFplayer is still functional.
Another area to consider is the SD card itself. It looks like the IC on this device is pretty fussy about the way the card is formatted and the naming structure. It needs to be formatted using a FAT16 or FAT32 structure, and naming of the files has to be done in a very specific manner. I am not an expert on SD cards, but I believe that it is difficult or impossible to format a card larger than 32GB with FAT32 and larger than 2GB with FAT16, so your card should be no larger than 32GB.
I found a thread on the Arduino Forum where someone had a similar problem as you, and they solved it by reformatting their card and implementing the file and naming structure described there.