It’s not quite self hosted, but Soulseek allows you to share share private directories with buddies. Soulseek might require a port forward.

Other than that, there are the many pasteboard solutions that have been mentioned. They’ll either require a port forward or reverse proxy (nginx etc.) to access outside the network though.

I might give this a try. I’ve been using the futo experimental for the jp input

Well because of this, I bought a new phone and am now using GrapheneOS. I’m hoping to last at least 5 years with this, and hopefully by the time I want a new phone, grapheneos will have made a deal to have more compatible phones.