p.p.s - I do get this warning in the gulp task:
‘layouts/tinymce.less’ wasn’t found. Tried - /Users/patrickdavey/sites/blog.psdavey.com/wp-content/themes/sage-8.4.1/assets/styles/layouts/tinymce.less,layouts/tinymce.less in file /Users/patrickdavey/sites/blog.psdavey.com/wp-content/themes/sage-8.4.1/assets/styles/main.scss line no. 19
which I “fixed” by removing less as per this comment