1. Open a new document with your preferred document profile (Film & Video, Art & Illustration, Web etc.).
2. Create an object and give it the stroke weight you want to use (and add/delete any other appearance attributes).
3. Open the Graphic Styles panel.
4. While holding alt, drag your object over 'Default Graphic Style' and drop (to replace it).
5. Save your file to C:\Users\Username\AppData\Roaming\Adobe\Adobe Illustrator 28 Settings\en_GB\x64\New Document Profiles
6. Use the new document profile you created for each new document, and it'll open with that default graphic style.