Gunna Bin The Lot GRR

[JAZZCNC]

Just on the spindle proper, if memory serves, it's connected to the spindle/power control board and the only power in says 36v and 18v with a 24v connection out to the YooCNC - stuffed if I know to be honest.

Ok I'll take an educated guess and say the 36V is for the spindle controller. So really answer to your problem is simple.? Replace the 24V PSU with 36V PSU and run them all from the same PSU.!!
Really you dropped a bollock buying the 24V supply anyway because the drives can handle upto 50Vdc and the motors will handle more than 24V, they will easily take 36v. The higher voltage will give more speed and torque so your Win win.

If the spindle board has run or start/stop signal pin then run this thru the Relay on the BOB and back to the board like Tenson says.
The BOB relay output is controlled thru M commands in the Gcode so it turns spindle on/off. If not then has last resort you could run the Spindle board power thru relay but if like MushT says it's flaky then it's risk you take.! . . . This is what the picture on advert going to spindle is badly depicting.!!

The spindle board will most likely have 0-10v connections which use the Step/dir pins to control speed/direction which you then take from spare output like A axis.

[Musht]

If its the original Yoocnc driver board for the 200W spindle on the 3040, it takes 220AC as marked on the board, it has a transformer mounted on board , think spindle, stripped down internals of s/h servo, is actually 90V.

If its something different , post a picture..