yep, Big Sur… I just purchased a refurbished LG ultrafine monitor so I hope it will be more compatible, the 4k with thunderbolt 3 connectors. My other monitor is in the shop checking to see if there is any problem with it. It’s an LG 27UK850-W 27″ 4k UHD IPS monitor. I connected it with the cables it came with (tried them all). I think I settled on usb-c. I also bought a dock and tried to connect peripherals to that. It only happens when I connect external displays. The projects are 4k. I’m not sure about mixing 2k and 4k. I was using a Dell monitor which used to work fine with my old macpro. I’m pretty sure I also bought a cable that was supposed to work with 4k monitors.