How do i make edge default to fullscreen

That depends a bit on how you launch the program. If you use the taskbar icon,

• Close down Edge.

• Right-click on the icon , then right-click on Microsoft Edge (near the bottom) and select Properties to open the shortcut's properties dialogue.

• In the Target box, at the end of the path you see there type a single space and then
  --start-fullscreen
  It should look something like
  "C:\Program Files (x86)\Microsoft\Edge\Application\msedge.exe" --start-fullscreen

• Click Apply and OK to close the dialogue.

• Click on the icon to test.

I did this, confirmed the change, restarted my PC and it still doesn't work. Using Windows 10 with latest updates.

Make sure you don't have Startup boost enabled under System at ​⋯ (Settings and more) > Settings > System and performance. If it is, any start-up parameters will be ignored because the browser is already running in the background.