I was struggling with this just today: Realizing I keep putting off writing tests for some modules because mocking a certain mega interface is a pain.
Your post gives me some great hints for things to try. Looking forward to experimenting with it.
Adding private members to my structs for testing was something the Go team forced me kicking and screaming to do when I worked at Google. I remember writing this adaptor for some internal network filesystem, and had an interface and two implementations: one that used the network for real, one that did everything in-memory for tests. I sent it off for code review and they were like "nope! don't do that in go!" and suggested an if statement in every method to handle the in-memory implementation. I was unhappy about it for months because it went against everything I knew about programming at the time, but like 10 years later, I appreciate that they were right. (I will say that everyone I explain this to has approximately the reaction that I had at the time. It is an easier pill to swallow when the person telling you not to do it wrote Go, but they don't have that benefit ;)