Open the file C:\Program Files (x86)\LibreCAD\resources\library\templates\empty.dxf
Set your preferred settings in Current Drawing Preferences and save the file.
Then, next time you open a new file, the settings are there.
empty.dxf is the default template, loaded when you start up LibreCAD.
Also, when you create a frame inside empty.dxf, you will always start with that.
You can have more templates e.g. for different paper size or orientation and use
New From Template to open them.
Armin
investing less than half an hour into Search function can save hours or days of waiting for a solution