From the “how could I have missed this feature for so long?” department:
GMail can schedule the sending of emails to a freely selectable future date and time. (I have no idea when that feature was added, probably a long time ago.). In the browser, just drop down the menu in the Send button:
According to cnet the GMail app has a similar feature, but I don’t use that app, so I don’t know.
Scheduled mails turn up in the scheduled folder and sending them can be cancelled there which moves the message back to the drafts folder, so you can edit it and possibly schedule it again.