So it's time to turn on my rig again but i've run out of working PSUs.
I'm looking to use a molex to 8-pin PCIE adapter like this one in the image below.
If the 3060ti only consumes around 120-150w undervolted do you guys think it's safe to use this?

You are asking for trouble.
If for some reason your undervolting fails and your card revert to default, they will burn out