Let it be known that the suggestion by @leroydouglas actually works. Thank you!
However, Apple completely dropped the ball here. Not only does the setting "Always" have the horizontal scrollbar obscure the resize grips but it also changes the behavior of the vertical column separator lines. In the worst possible way, one might add.
When "When scrolling" is activated, there are no grip points anymore but only thin vertical lines between columns that are grippable everywhere. Apple got rid of the grip points and not allows users to click-and-drag anywhere on the column separators.
By contrast, when "Always" ist selected, the column separators revert back to their old behavior. They become thicker and are no longer clickable everywhere but only at the grips – which are obscured by the scrollbars.
In essence: Exactly, when you would need the "resize everywhere" functionality, Apple removes it and introduces a UI bug which makes resizing impossible.