For now the best way I have found is to put the content on my website, in a folder protected by a password and served by TLS only. Anyone can receive it, I control expiration and access, even as a sender I'm using extremely simple, reliable software. It's not like I need to send a lot, it just works.

The hardest part is being able to have a domain name that links to my machine. After that Caddy makes everything extremely simple.