The import path for lv_conf.h was the only change I made to the stock LVGL library.
The error you're getting seems to be a limitation with Windows. Here is a description of the Argument list too long error:
This issue often arises in projects with a large number of source files, overly long paths, or excessive arguments passed to the compiler. It's a common issue on systems with limited argument list length, particularly on Windows when using tools designed for UNIX-like environments.
Searching the forums for Argument list too long error shows this being a problem for other projects with large imports.
Are you able to continue your development on a Linux machine? That might be the easiest solution.