SAI Transmit Configuration 1 Register (TCR1) p.1987
Correct me if I'm wrong but I was looking for I2S_TCR1_TFW(n); // Transmit FIFO Watermark in \hardware\teensy\avr\cores\teensy4\imxrt.h
I do see:
#define I2S_TCR1_RFW(n) ((uint32_t)n & 0x1f) // Receive FIFO watermark
I believe this supposed to be:
#define I2S_TCR1_TFW(n) ((uint32_t)n & 0x1f) // Transmit FIFO Watermark
Configures the watermark level for all enabled transmit channels.

Looks good on the SAI Receive Configuration 1 Register (RCR1)
#define I2S_RCR1_RFW(n) ((uint32_t)n & 0x1f) // Receive FIFO watermark