The most common action I have in Penultimate is moving up to select the 'Erase' button and then moving up again to re-select the 'Pencil' button. I do this over and over as I'm writing and then fixing little mistakes along the way.
The Procreate app has a fabulous option where I don't need to keep switching between eraser and pencil. Instead, I can use my finger as the eraser. It would be great to have the option to use my finger for erasing and pencil for drawing. No switching necessary.
Here's a write-up of the feature as it exists in Procreate: https://www.igeeksblog.com/how-to-use-your-finger-as-eraser-on-ipad/