Tinkering with the SKR Pro tonight, teaching myself pin (re)mappings and the like. @ByronM’s documentation was incredibly helpful for me as it helped me understand what the changes I was making did, and how they worked.
I remapped the Heater 0 output to be the new Spindle M3/M5 control, disabled the PWM function, and connected a solid state 40 amp relay to the Heater 0 output. M3 engages the relay, M5 disengages it now. Next steps are to configure the E-Stop trigger so I can mount a button on the table, sort out the Z-Probe connection and possibly move it to a new pin for consistency with connections, and finish crimping the Hitachi locking connector to Dupont connectors to ensure a solid mechanical and electrical connection at the board.