Have you seen the Quick Menu? It's faster than double tap stylus since you can do quick flicks with items in the first ring of menu (8 quick-flick items + 16 others!). It's faster than procreate since there is no need to wait for the menu to appear (when using the "Finger Function" gesture to initiate it).
If you really like double tap stylus to get to eraser or smudge, then keep it on and use Quick Menu to get back to brush - no interface needed!