Thanks for the advice.
Here is the conclusion what had to be done. Please take into consideration that these CSS rules are somewhat general and could break stuff in other places unless you make them more specific.
V9 panel operations are inside <button> tags instead of <a> tags like before. Thats the major structural difference I can find.
V9 also includes a CSS "gap" between the buttons, make sure to set that to 0.
.aw-horiz-top-toolbar.k-toolbar-md, .k-button{gap:0;}
The overflow menu would trigger around 25% width. Using this CSS prevented the menu from appearing:
.aw-horiz-top-toolbar .k-toolbar-overflow-button{display:none;}
Now it works to have full width (33.3% width on each of the 3 buttons). There is still about 10px of inexplanaible space to the right. But it is only a minor annoyance.