I've created a visual example using html and css here: https://www.w3schools.com/code/tryit.asp?filename=GIR6MAODWHYO
Please note the following is my opinion based on my design experience.
As you can see in the example, when you hover over the bar the contents triple in size with a majority of the buttons no longer being visible. When a toolbar is overfilled like that in OPUS it instead adds a small arrow button that opens a submenu containing the rest of the items.
Essentially, imagine moving your mouse to an icon on your toolbar, only the instant your mouse crosses over to the toolbar that icon disappears or shifts position. That does not sound like fun for most users. You now have to relocate that item and potentially dive into the submenu to find it.
I'm not sure what advantage labels on hover would bring either. It's not space saving and you aren't getting the benefit of being able to read the labels until you actually hover over the bar. Typically you look at the toolbar first and find the item you want to click on before moving your mouse over.
I think a better solution might be a hotkey to show all tooltips for toolbar items of a specified toolbar. That would satisfy both your points without any notable drawbacks.