I use the Dell U4021QW as a second monitor on a Mac Book Pro 16" with M1max, connected via the supplied Thunderbolt4 cable. I have connected Ethernet and USB-A sticks to the monitor. This works with full 5K resolution and the Mac is also supplied with power perfectly. I don't know how long the connection cable should be, but it is important that it is a Thunderbolt 4 cable (i.e. not just a USB-C cable, which has the same connectors).

A small disadvantage with MacOS 12: If you put the Mac into hibernation mode, all open windows are on the main screen (the one with the menu bar) when you wake up again - a bit annoying and that's why I always switch off the Mac, then all open windows are displayed again where they were. I haven't tried out in detail how this works with MacOS 13 Ventura and the new Stage Manager. There are some settings that might solve the problem.

Basically, I am very satisfied with the monitor. I really appreciate the "curved" and the Ethernet connection in the monitor. The Apple Studio Display doesn't have either of these. The Dell has an anti-reflective, slightly matt display (similar to the Studio Display with nano-coating). I find that great for working. However, if you want to watch films, a glossy display, like the normal Studio Display, would be a little sharper / clearer / more brilliant.