How do you make complex designs faster?

Branching scenarios aren't exactly something where a template or checklist is going to work (other than perhaps templates within Storyline or for collecting SME information). A template wouldn't make any sense in Twine.

If you're looking for a learning strategy that can be the same every time you create it, and therefore a template would be helpful, then you're looking for something that isn't a branching scenario.

The reality is that every branching scenario project is different, and the branching structures and approaches used depend on the audience, learning goals, and other constraints. There's no "this is the magic easy button." There's a bunch of "here's some considerations and factors to weigh for different approaches." If it was easy, there wouldn't be enough content for me to teach a 2-month-long course on it. :)

I have a series of 9 blog posts that explains my process of creating a branching scenario from start to finish. What's on my blog actually is my process; I really don't use templates.

You might also find Patti Bryant's presentation on scenario mapping documents helpful. This is a way of collecting and organizing the information from the SME. My own interview process tends to be more fluid than this, but you might find this kind of structure easier.

You might also look at Kimberly Goh's approach. Her examples are more "interactive stories" than true branching scenarios, with a very limited branching structure. She advocates for more of a linear/fake branching/gauntlet structure (she calls it "optimized"). While I disagree with her conclusion that the limited structure is right for most situations, I 100% agree that it's easier to manage. If you're looking for plug-and-play (kind of) branching structure, then this is a better way to go. This might also be a better starting point for you than to do a true branching scenario. https://www.youtube.com/watch?v=e44I1CLlxPk