It looks like this has been proposed a few times over the years and I still desperately need it! As a broader idea, it would be helpful if Toolbar customization could include many more options than what is given. But I personally add to my outline (which currently takes multiple clicks to do) far more often than I ever use a ruler (which is a toolbar option that I have never used).

At minimum, it would be helpful if the Outlines panel at least had a one-click to add option when it is open instead of having to click the three dots and then click "Add outline." Overall there are just far too many steps involved in creating a new heading.