Your analysis is correct in that converting sources as you describe is an incredibly labor intensive task. I have been working on the task in my database for years, and I am not even close to be being done.
In my case, the issue is not converting from a third party software's version of sources to RM's model. Rather, it is converting from RM's Free Form sources that I had already entered by hand into RM to using RM's source templates. Part of my project has been to move all sourcing data into RM's Master Source - the yellow area on RM's data entry screens for sourcing data. I'm using only the yellow area rather than the green area so that any correction I make will be applied immediately to all the citations for the source rather than requiring me to chase down each citation and make the same correction multiple times. To that end, I developed my own source templates, similar to RM's except that they place all the data into the yellow area of RM's data entry screen for source templates.
RM8 will include a feature which I think will be called Shared Citations, although I am not sure of the official name for the feature. The feature should allow a given citation actually to be shared multiple times rather than being Memorized and Pasted multiple times. After RM8 is released, I will need to examine this feature very carefully to see if I wish to start using it or if I wish to continue doing my sources the way I have been doing them.