The GitHub user interface will default the option to the last option selected by the user for the current project. This solves the issue of not accidentally using the wrong default, since after the merge is done correctly for the first PR, subsequent merges will have the desired default.
This solution isn’t perfect. If a user makes an intentional one-off merge of a different type, they will need to remember to set it back for their next merge, as the default will have changed to what was intended to be a one-off deviation from the norm.