The subject line is most of the email
Most recipients decide whether to open from the subject line and the first few words of the preview, on a phone, in a list of forty other emails. Everything you wrote inside is downstream of that one line.
Length matters more than it sounds: a phone shows roughly 40 to 50 characters, so a subject that runs past that is read as a fragment. Writing several and picking is faster than trying to write the right one first.