Use ～ことだし when you want to offer a reason as an already-shared, self-evident circumstance, and then propose or conclude something on that basis — very common in meetings and email when softening a suggestion. Unlike ～から/～ので, it doesn't argue the point; it nudges ("seeing as we all know X..."), so it pairs naturally with ～ましょう, ～たらどうか, ～ことにする. It sits in polite-but-not-stiff register: fine in business email and discussion, too casual for contracts or official notices.