Updating SPI transfer16 to create Transfer24
You can't.  The teensy 4.0 has hardware support for 8-bit, 16-bit and 32-bit transfers. Not 24-bit transfers.
But at 600MHz the overhead of making a function call is negligible anyway - certainly compared to the speed of SPI which is at best about 30x slower than the CPU operates at.
If you want high throughput you won't be wanting to use the SPI library anyway, instead you'd be using DMA to transfer your data with minimal CPU intervention.