the on/off button only electrically connects wall power plug to psu.  when you only connect psu to video card, that does not turn on psu.  unless you connect the psu to another pc (or short two pins in its 24 ATX) and turn it on, you will not have any power out of it.
if i understand correctly cards over gtx1650 won't work due to lacking bios support. what about 1070 and 1080? CAN I USE A 2nd standard desktop PSU not connected to motherboard BUT only to power the graphics card to power a 1070 1080 card if the card size fits the x51 ????? And what about amd cards? what generations are supported by bios if power was of no concern??? I just think this would probably work fine.....   
X51 R2, 330W PSU, graphics card upgrade, external PSU?