I have the B550i which uses the same bios. Undervolting the 5800x3D is possible, I do it using PBO Curve Optimizer in the Bios.
Upgrading to the x3D from the 5800 does not make any sense unless all you play is graphically light games at a very high refresh rate. I upgraded to 5800x3D from 5600x last year and I barely noticed any difference in gaming tbqh, but then again most of my games are graphically intensive and I play at 1440p so my framerate is GPU limited and CPU doesn't really matter at that point.
