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.
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.
Having seen many of Kaze's videos on N64 development, I've learned that the N64 has like 4x the processing power it needs compared to its memory. In hardware cases like that the trade-off of computational power and memory memory savings gets you some nice performance gains.
Mirror's Edge is still beautiful to this day, despite being 17 years old