Happy to help Stuart.
Do email me at C.J.Sangwin@ed.ac.uk if you need specific help with STACK.
I have to be honest. We basically ignore the grade sync via LTI in Edinburgh. Course organisers use the Moodle gradebook, and we upload a marksheet with the amended marks direct to the Edinburgh assessment database. There are always edge cases needing human input, and we upload more than one grade normally with individual quizzes feeding into different "formative quizzes", "assessed quizzes", "Online exam" assessment components in the official university database. I'm happy to talk about the course enrolement plugin https://github.com/maths/enrol_lticoursetemplate but the original goals were to get this working in a way which asked the minimum from the Blackboard side!