"Need" is a strong word - "optimal" would be IMO more accurate, but you're not wrong, to get max performance and avoid a lot of issues (probably), ATX 3.0 will be necessary. I have a 1300 G2 PSU in my 3090ti system, and a Seasonic Prime 1300 platinum in my 4090.
I'm in the same boat as you, waiting for the ATX3.0 Seasonic to come out.
One thing I'll say is a huge must have is Cablemod's 90 degree adapter for the ridiculously designed plugs/location on the GPU. What a PITA, some propeller head engineer had to have come up with that idea.