nncp [1, 2] is probably the best Sneakernet tool I've found. It's very UNIX-y which makes it pretty hard to operate if you're not technical but would also make it pretty easy to wrap around with a UI. You have to explicitly add a list of "neighbors" to your configuration and you can send "packets" either by spooling to file or using a TCP/Noise connection. You can also send data hop-by-hop and is e2e encrypted.
...and it's close relative NNTP. There was a whole distribution structure built out of intermittent data transmission. We've had the tools for this stuff for a long time, we've just switched to centralized, always available services because that's easier to build a company around.
Usenet is fascinating. It's said that communication system is doomed to reinvent half of Usenet the same way every programming language reinvents half of Common Lisp. Hacker News is a itself a worse form of Usenet. And yet almost nobody uses the original.
(Go sign up to eternal-september.org if you're interested)
While the network of Usenet is pretty fascinating, the NNTP protocol is pretty bad. It's a stateful protocol which makes it challenging to write clients for and the protocol itself is textual and chatty which makes it fat on the wire. ActivityPub is a much better "modern" manifestation of the protocol and its capabilities, though unfortunately the actual AP standard is barely glanced at and most people write code for Mastodon's implementation of AP with a minority focused on Lemmy.
Feel free to write a new protocol implementing NNTP semantics with less cruft.