Improve the toolbar
When there is room, take out items in the overflow menu and move them into the toolbar as icons with text next to them and squeeze in other items by removing the label. Hovering over an item that only has an icon would bring up a tooltip.
You might be wondering. What would this look like? This would look like Office 365's simplified ribbon or OneNote UWP's ribbon - except without the tabs. It would still have the overflow menu for items that do not fit in the toolbar but it would get rid of that gap and create something more responsive. Do NOT take this as "remove the undo button when it's unavailable". It's a good idea to keep the undo button because it tells users that it's something they can do and when they can use (keyboard) shortcuts.
What tool bar? How to I create bullets inside my e-mail message?