My broad intention is to allow the site builder partially declare
some config values they want but which might already exist (so cannot
just be deployed via config/install/settingname.yml).

For the case of list data types, overwriting the list rather than
appending to it seems most appropriate: otherwise, each config merge
will lead to the entire set of list values being re-appended. I think
cases where you want to append to an existing list ore best dealt with
in PHP by just loading the appropriate config key and manipulating it.

Also adding a dependency on php >= 8.1.0 due to use of array_is_list().