Seems to me a .tpl file is exactly like a .pln, except that when you select the Save command, it will prompt you for a file name etc like when you Save As... a .pln.
If you use a .pln as a base for new projects, you will need to remember to use Save As, or you will overwrite the original.
So a .tpl is safer.
The default object (in this case, window) settings are stored in the library, not in the .pln or .tpl. You should save your changed window as a Favorite.
AC4.1-AC26SWE; MacOS13.5.1; MP5,1+MBP16,1