I don't have any solutions, but I have some ideas to explore.
I use BetterTouchTool to map 3-finger clicks as middle-mouse-button. It can interact with different mouse, trackpad and keyboard input.
I just started using Hammerspoon and Karabiner-Elements so that I can use Caps Lock as a modifier key, for my own custom keyboard shortcuts. Karabiner-Elements is able to remap keyboard keys into very special functionality. And Hammerspoon can do all sorts of awesome stuff to control your Mac.
I do like you and use middle-mouse on my Wacom tablet to scroll through webpages. I wonder if this behaviour can be triggered by a special key-stroke in Karabiner. For example, whenever Caps Lock is pressed down, clicking is temporarily disabled and it will act as if you are middle-scrolling. These programs are difficult to configure, but I think they might have all the functionality to achieve this.