Test board on box with cpu, 1 stick ram, cpu cooler and harddrive to see if it boots. Just bare mininum to rule out something. Anyway you can test connectors from psu by jumping 24 pin to see if they get correct voltages?
Maximus XI Formula, I9-9900k, Phantex Evolove X, Seasonic Titanium 850W, Custom loop PE360+SE360 Rad, G.Skill Trident Z F4-3200C14 32g, Nvidia Reference RTX 2080 TI, Samsung 970 EVO Plus 1Tb, Windows 11