Interested to know why the ultrawide made such a difference, compared to 2 side by side?
Depends on the UI you use. Gnome and tiling window managers should do it just fine. I prefer the macOS keybinds (including where full screen becomes a new desktop, combined with the "Magic Touchpad" gestures).
With regards to keybindings, one thing you can do is get ctrl/meta/alt in the same order as your Mac. Personally, I've swapped ctrl and meta on my MBP while I am using VirtualBox (with Kali) because that way when I'm in the VM I get local consistency. I usually SSH into the Kali machine from the MBP anyway.
With two 1920x1080 displays you can fit even more (or better 1920x1200). Also, what is the benefit of an ultrawide 2560x1440 compared to 2560x1440 or 2560x1600?