Now I could be wrong on this but I do not think there is a built-in default way to do what you're attempting to do. There may be a third-party application to do so that will work with O365 to help you but I've never used them and you could probably do a search to help you. for example, I found https://www.codetwo.com/
I saw no options in the mail flow rules that might seem to indicate you can do this for outgoing email at all. Individual users can use Outlook rules to try and search for incoming rules. I know people have modified inbound subjects to remove appended subject lines from exchange when replying to an external email.