Multiplexing SPI "Chip Select" lines (no replies)

Hello everyone.

I have an Azteeg X3 PRO motherboard, with a Viki 2 LCD display. Currently I have 6, TMC2130 stepper drivers installed. All of the drivers are using the SPI interface to set the parameters. I would like to upgrade to 8, TMC2130 stepper motor drivers. But I have ran out of I/O pins (that are provided by pin headers) to control the additional TMC2130 chip select lines. I have a very good understanding of electronics and hardware. So I figured a part that worked great in the past, would still work today.

I am talking about the 74HC138, 3:8 decoder. It takes a binary input value (0 to 7), and makes the corresponding pin LOW, perfect for chip selects.

I am no good at C programming. I have the basics, but nothing good enough to alter Marlin. So I was wondering, would someone be willing to give me a hand to make Marlin output a binary value (on 3 I/O pins) that represents what TMC2130 chip select pin should be active?

Maybe this could even be a feature that could be added to Marlin. Or is a feature, and I just didn't know about it.

Thank you for any help or input you can provide.
AJ