A comment by Alexander
If we're talking minimalism (and Go, of course), https://viric.name/cgi-bin/filegive/doc/trunk/doc/home.wiki has you covered. With `filegive theFile`, you get a one-time URL (also printed on console as QR code) for downloading the file. With `filegive -R`, you can receive files. Don't trust the ephemeral self-signed certificates? Supply your own CA certificate with `-ca`. There's also some options for NAT traversal which never worked for me (networks are cursed, see https://xkcd.com/2259/) but might for you --- after all you seed to know how to work with rathole.