My 6900xt showed up today! and it works!
My questionable ideas have also worked (so far):
adding a PCIe expansion slot to my itx mainboard by sacrificing the wifi card
and running a 6900xt, Vega 56, and a 4790k off a 7 year old 650w* power supply
While I was able to eth mine with no problems on that power budget, to game & mine at the same time I had to turn power sliders to minimum**. This lost a 100 points off the 3dmark score ( 12200*** to 12100 ).
* totally recommended by everyone! I do not see how anything possibly could go wrong ... 210-GQ-0650-V1 ftw
** -50% on the vega, and -10% on the 6900
*** it is paired with a 4790k from 2014, delighted with what I have
🙂
lootbox!:
installed in a questionable manner:

1x wire to each card ... anyway that itx mainboard looks tiny in this considerably larger case