Hi! I got same problem, I have 3 diferent logic boards with Radeon Vega 48, all the same issue, led2+led3 and fan spinning. Ram, using default 4GB+4GB not starting, tested with 4x32GB Crucial compatible and same. If I do this with the default logic board, it works with the same iMac 2019 27". Did you use Command+Alt+P+R to reset the RAM?
I have tested also with i9 9900KF and got the same results, can be possible that the i9 Radeon Vega 48 Logic Board matches the CPU? I mean, I bought months ago a fresh Logic Board+i9 9900K CPU and I replaced without any problem. This 3 boards comes with Radeon Vega 48, two for i9 and the 3rd for ani5 3,7Ghz, without CPU. I have replaced the PSU, the same, but when I fit again the original Logic Board (i5 3,5Ghz 580RX) systems starts perfectly, no issue, all leds working, I mean that Display and connectors are ok, does any one replaced their i9 Radeon Vega 48 CPU or similar?