S/MIME seems to be built into most major email software (?) and is surprisingly usable. Not quite as easy as "it just works" in popular messengers, but still good given the nature of email.
The most difficult part was getting a signed certificate. There's only 1 free provider left (apparently) and the payed one I've tried was a worse experience. This is a hindrance, but something that could be solved if usage was more wide spread in the first place. But how come it (seemingly) never got off the ground in the first place?
