"frame off" is a bit of a misnomer for E9's
There is a front and rear subframe but no major chassis frame. Maybe better to call is a bare metal restoration.
In regards to your question of why it's so complicated when cars are all basically the same ...
(1) Many restoration shops are run by guys with passion and deep hands-on experience, but who are not good business/operation people. this leads to needless operational complexity. Things like detailed job specifications, change control process, and professional project management is what enables engineering companies to design and build tremendously complex new systems on time/budget. A small application of such principles (as you well know as a business owner), could make a big restoration shop much quicker and more efficient too.
(2) specialization: you've seen that while there are really NO E9 specific items when it comes to body and paint, there ARE a lot of E9 details regarding trim, mechanical, year on year changes etc. Plus there are period-specific specialties such as electrical systems, suspension, wood, trim, etc. For a shop to do everything efficiently and with high quality they generally have to have some specialization to a limited range of cars and periods, and subsystems.
John