Yes, we buy 5K+ at a time, and have had a hard time since before covid. It has not been easy to find. I would love a comparable audio solution from an abundant chip, but have not been able to I find anything better yet. I would gladly pay...
It's the modified SD library by teensy. You can open SD_T3.h I think, and edit two lines, one for memory one for enabling optimized/buffered. I assume it's similar to what you are doing, but I get best results with files with no header (RAW).
I play up to 8 files at a time with an average of 3. All from SanDisk high speed cards. It's OK but s bit choppy with optimized SD library and extra spi SD card buffer at 50 instead of 7.