May I ask as to why having a full size SD slot is important?
I'm genuinely curious, full size SD realistically hasn't anything to offer a digital audio player that microSD doesn't, and with microSD UHS-II speeds are comparable unless you're looking into the world of ridiculously expensive cards for photography professionals, though I'm given to believe that compact flash is still a widely used format in that area.
The two technologies (digital audio players, and full size SD) didn't really overlap much. mini/microSD (or TransFlash as it was called, and sometimes still is) was becoming a lot more prevalent during the time when flash based digital audio players were becoming more common.
In fact, I am only personally aware of two devices which were reasonably common (though still fairly obscure) that have full size SD, one of them (the Cowon D2) has already been mentioned, and the other (SanDisk Sansa View) hasn't received any developer interest at all barring some firmware disassembly to confirm the makeup of the device, and signature verification (to make sure arbitrary execution with a modified firmware was possible), and that was some seven or eight years ago now.
[Saint]