The fact that it was all one physical monitor may have further confused the OS as a sibling comment mentions.
The solution was to sell the monitor to a Windows-using architect friend and buy a different panel with a resolution MacOS supports. She has a macbook too but it's the fancy one with discrete graphics which can drive 5120x1440.
The value proposition of MacOS to me is that I plug things in and they work. Any fiddling beyond that destroys the benefits of using this platform. I'm willing to iterate on hardware until I find something that works.
