How can I set a consistent font for sent messages in Apple Mail Mojave?
People who send plain text mails are heroes to me. There’s no downside there since it respects everyone including screen readers, people who have a font and size preference, etc...

There is a real cost to rtf bloat and html and you are seeing some of that pain here.  My solution is to eschew bu to know some people look down on that as plain text can be rendered horribly as well by some Mail clients. 

Sadly, you have to read the room for your intended audience and choose an authoring tool that is copacetic. A third party mail app might be needed if no one has a style tweak that lets you keep the styling features you know you need to communicate effectively. Outlook on Mac does a decent job of hiding that the mail came from Apple if you have the desire to try that.