As you suggest, one of the nice things about being an extreme source splitter in RM is that you can prepare an RM Master Source for use as a distinct step, separate from the data entry for people, events, places, etc. For quite a few years after becoming an extreme source splitter, that's the way I did it. For whatever reason, my process for creating a new Master Source has evolved into something similar but not quite as distinct. For example, suppose I am creating a new Master Source for death certificate.
From Death fact, I will tell RM to Cite An Existing Source, even though the source I'm about to cite doesn't exist yet. This step takes you from the Edit Person screen directly into what is essentially Lists>Source List. From the Source List, I will move down to an existing Master Source for a Death Certificate and Copy it. The copy will be identical to the original except that the name will have (Copy) appended. This (Copy) is the "existing source" which I will site. Having an existing and correctly entered source to work with is much easier than creating a new Master Source completely from scratch. So I will edit the newly created source as needed and rename it as needed. At that point, I am done with the citation for the Death fact. But the same citation usually also serves as evidence for other things like the person's name, their birth date, and their parents. So I will Memorize and Paste the newly created Master Source to these other places.
It may or may not be obvious from this description, but one of the most irritating part of this process is simply navigating to the the desired Master Source in a rather large list of Master Sources. I hope this is much improved in RM8. In any case, if I create a new Master Source by copying an existing one and editing the copy and if I do the creation from Lists>Source List, then I have to do said navigation in the Source List twice. But if I do it as described from the Edit Person screen, I only have to do the navigation in the Source List once.