Problem finally solved. I suspected HDMI being the issue and that was it. Dunno why but that was the issue. Bought a new card (same RX550 chip) and connected via DisplayPort. No more corruption at boot texts and 60Hz update rate at 3840x2160 resolution.
Apparently there’s some issues in Radeon’s drivers related to HDMI. Both displays supported 3840x2160@60 via HDMI but neither worked at 60Hz but 30Hz at max.
Anyway, thanks to everyone whom tried to sort this problem out.