Normally we strongly advice against overriding the TinyMCE CSS to tweak the editor. The proper way to change the visual appearance is to make a skin. That said, there is always exceptions to a good rule and moving a couple of toolbar buttons to the right side of the toolbar is one example of such an exception.
Since the toolbar is using flexbox it's pretty easy to just add one simple CSS override to move a toolbar button group over to the other side. However, there are some caveats:
But if you know what you are doing and accept the consequences, this is how you move buttons!