-u is the update version.-i is the input file.-o is the output file.
You need another parameter to tell Spine what operation to perform on the input and output files. To bring skeleton data into a project file, you want to use import. I don't remember if 3.6.53 supports the import CLI parameter. If it doesn't, you'll have to perform that step using the editor UI.
Your second command looks OK, assuming you have an export-settings.json that does what you want. However, why would you use 3.8.93? You should use 3.8.99. Even better would be to use 4.1, as there are years of development and thousands of fixes and improvements since 3.8.
