By changing “Space between items” do you enter a numeric value or “auto” (in advanced layout settings it’s “Spacing mode › Space between”)?
“Auto” is what you need.
This can also be achieved like this: wrap the center part in a frame and set the “Fill container” horizontally, and then change the constraints on the objects inside that frame.
What if you wanted to keep the vertical alignment? I have a series of elements that have unequal horizontal spacing as well as an uneven vertical alignment. Using auto-alignment snaps the elements into alignment. Any way around this?
I realize this scenario is somewhat unusual, so I’m not expecting that it can be done easily. I don’t miss XD but it was good at this type of thing.