[McAroni]

You could save every preset for your pen or other Tool in the favourites. After setting up your pen Go to the Star ar the top panel and press the plus Icon. You will have access to your saved favourites there, or through the command Bar. 
If you have an spen you could sign the Button to switch to the eraser, and if you release the Button it switches back. This is in my opinion the fastest way to access and leave the eraser