One tricky constraint is that a "simple URL" isn't big enough to hold a full-entropy encryption key. So your security must either come from PAKE (like magic-wormhole and friends), or from the good behavior of some intermediary. And PAKE requires a peer who knows the plaintext and will only execute the protocol once, which means it really needs to be the person you're connecting with and not an intermediate webserver.
I think it's a 2-out-of-3 trilemma: end-to-end encryption, short codes/URLs, offline/non-interactive workflow: choose two. Or framed differently, if you require proper encryption, then either the code/URL must be long enough to hold the full key, or you must use an interactive (PAKE) protocol which means both agents must be running at the same time (babysitting).
Your last point is an interesting one: we could build a form of magic-wormhole where the sender's CLI waits, the recipient gets a URL, the URL points to a web page which performs the client side of the protocol. The server wouldn't host the file, just the decryption agent. Basically wormhole receive in a browser. That would match many of your goals.
However I'd be hesitant to do this with magic-wormhole because it opens up a vulnerability we don't currently have: the web server hosting that page could silently swap out the JS with a version that retained a copy of the plaintext, perhaps only when the browser is coming from a specific IP. You can't audit a webserver for consistent+correct behavior the way you could with e.g. the contents of a Debian distribution.
That said, the usability gains of the recipient not needing a CLI tool installed might be worth the tradeoff for some folks.
(I'm the author of magic-wormhole)