I will look closer at this in the morning, but yes, that is what I am hoping to do.
Additionally, you may want to look at some pin re-mapping based on what @ByronM did here for enabling M3/M5 spindle control. The SPINDLE_LASER_ENA_PIN function is missing in there, and needs to be explicitly called out.
I did so using Byron’s post as guidance and was able to turn Heater 0 pin PB1 into my Spindle on/off function. Combined with the Spindle button on the TFT35, I am now able to directly control a solid state relay for this. While this pin mapping may not be ideal for everyone, for pure on/off control of the spindle, it works well to avoid any potential overcurrent issues with the “coil” of a relay.
//
// M3/M4/M5 - Spindle/Laser Control
//
// use P1 connector for spindle pins
#define SPINDLE_LASER_PWM_PIN -1 // Hardware PWM
#define SPINDLE_LASER_ENA_PIN PB1 // Pullup!
#define SPINDLE_DIR_PIN -1