Currently, the only way to do what you are asking is how I described, whether you do this in the GDL editor or through XML export/import. In any case, you will have to alter every Object in your Library that you want to set the defaults.
I have setup numerous offices with custom templates and standards, and one thing that I tell my clients is to adopt and follow the Pen Settings (at least similar pen weights), as well as Fill, Linetype, and Material Settings, that are provided in the default ArchiCAD template file. This will alleviate issues that you are currently experiencing.
David Larrew, AIA, GDLA, GSRC
Architectural Technology Specialist
a r c h i S O L U T I O N S
WIN7-10/ OSX 10.15.7
AC 5.1-25 USA