A complete UI overhaul of Rhino would be a much welcomed move

Same is true for a long left click.
Means a long click opens the linked toolbar.

Or correct: A long mousedown opens the linked toolbar.
After that, you only have to release the mouse over the wanted button.

Right, there is no need.

A new UI feature based on timing could be no click at all to open the linked toolbar.
Just waiting a certain time over the button, let’s assume the double tooltip delay.

And of course the automatically opened toolbar must vanish automatically if you move the mouse to a different place.
And after selecting a button from the opened toolbar.
Simple as that.

See the video from Xara here:

The same system could work with the toolbar tabs.
It did once long ago, but didn’t work right.
It was replaced by mousewheel action.
Pascal will remember.