I wouldn’t say that SGF is “loosely defined”. In fact, the spec that you link to (SGF file format FF[4]) is quite thorough and precise. There have been changes to the format over time, but these changes have been well-documented and there is even a guide on how to properly convert files up to the latest version (see SGF FF[4]: Converting old SGF files to FF[4]). The latest version FF[4] was introduced in 2006, and overall the standard seems quite stable as it has not been updated since then.
The “many different varieties” are not due to the SGF standard being “loosely defined” (which it is not), but rather are due to various applications not properly complying with the SGF standard.
I’m not familiar with the behavior of OGS SGF upload/import (since I have not used that feature), but I have seen that OGS creates SGF files for export/download that violate the SGF standards. Here is a separate thread where I document some of these bugs: