I explored and collected GPS traces for another substantial chunk of this walking route on Saturday, covering around 19 miles from Marlow to West Drayton (and so linking up with a section along the Grand Union canal which had already been tagged). I've now added this section to the route relation.

I found that from Cookham onwards the Shakespeare route follows the same footpaths as a local county council waymarked trail - the "Beeches Way", so I've created an (almost complete) route relation for this walk into the bargain:

osm.org/browse/relation/974619