@tinnut Set each GPIO you use as a slave select to be an output and set it high

some info to do it in terminal other then scripts and code

https://raspberry-projects.com/pi/command-line/io-pins-command-line/io-pin-control-from-the-command-line

or use a library as WiringOP

or even better buy a Orange Pi for 10 euro and check your pins of your opensprinkler pcb most likley you only have it to turn around RPi is right to left and a OPI is the io pins from left to right .

Get Armbian and have mainline kernel and up to date linux distro

have fun