In the Beta a new toggle feature was added for various tools using brushes where you change the functionality for alt+R click+L click. I did notice however that it doesn't support a few settings as seen in the video:
Desktop 2020.01.19 - 05.04.25.01.mp4
For the Brush Tool you are not able to change Opacity and Flow with this shortcut, which I think are pretty important settings to change on the fly as well. The Smudge Tool is also missing two important settings like Strength and Flow from the toggle options. I think there are more examples of this for some other tools as well like Blur, Sharpen, Sponge, and more.
P.S. Could you also increase the contrast of the UI elements for the shortcut please? Notice how much they blend into the painting in the background, which makes them a bit hard to see.