Both an extra monitor and an ultrawide add more space for windows, but the naturally partitioned nature of a second display combined with virtual desktops also provides low effort organization.
So in my typical usage, secondary/tertiary/etc windows go into purposed virtual desktops on the second monitor — e.g. one virtual desktop for documentation, another for chat, etc. This allows me to swap out the sets of less important windows independent of the main display’s workspace, with practically zero management overhead (just assign apps to virtual desktops during initial setup).
The extent of third party window management I use is Moom, which I use to snap maybe 3-5 windows across my entire workspace into specific locations/sizes. Something that emulates a Linux tiling WM like Amethyst would drive me insane.
It's probably the simplest one. Draw the shape you want on a grid, assign a global hotkey to each shape (fullscreen, top half, bot half, left, right, etc.), press the hotkey to move current window into that shape. Press the same hotkey again to move it between monitors.
It's funny how you get so used to your own dev environment and all the global hotkeys you might have and then completely take it for granted until you use a computer with it.
How is that solved by adding a monitor? If I used two (I don't any more) I would still use a (better than macOS default) window manager.