Awesome write-up. Interesting about the amp differences and revisions.. strange they did that.
I was wanting to do all this, but had such a hard time getting the proper wiring diagrams for the 19+, and used HUs were hard to come by in Canada (new was way cost prohibitive up here). I decided to just live with my 17 HU considering I picked up a dd with CarPlay, and my STI has become the weekend car.
Anyway, thanks for being so dedicated to this. I’m sure it will help many others. Great job.