I'm not speaking from experience, because I've not used it, but I think the most out-of-the-box compatible ultrawide is the 5K ultrawide, that is essentially a 4K display made widescreen. Here is an example display. Here's another. [video]

You can use a 1600p or 1440p widescreen, but again, you'll want to use BetterDisplay or SwitchResX to enable HiDPI, or otherwise it will look way worse than when connected to Windows.