[troydavis]

If you’re willing to move to FastMail, FastMail’s native iOS app has this. You can rewrite the from address to use any username, any real name string, and/or any subdomain within your domain. There’s no additional setup, you’re just able to edit the from address in the compose screen.

[MerelyMortal]

I use the Fastmail app on Android, and it works very well regarding this, it sounds like its iOS app is the same, great!

[thewavelength]

Thanks, good to know! If I can't find any other solution I will think about this.