I've found that the best "split" keyboard is simply to use two decent keyboards on the same computer. When I am in a situation where I'm doing tons of typing (not that often anymore) I'll have the two keyboards a bit more than a body width apart. Unfortunately macos doesn't apply modifier keys (shift / meta / ctl) from one device to the other (IE if I press left shift on the left keyboard I don't get a capital L if I press it on the right keyboard). Windows did this just fine.

For just general sysadmining around, one keyboard's been fine for me. Frequent breaks, standing up and moving, etc, have much more of an impact on fatigue than the quality of the keyboard. That said, I've found that the 1990s model M4-1 to be an absolutely spectacular keyboard.