Dumb question: can anyone host a i2p site on a personal device? For instance, if I leave a PC or old phone connected to the network, how easy/hard is it to configure so I have a working site accessible to everyone on the network?
Anyone can host a site. Just keep it up for like a month without a pause so that it can be discovered.
Then go to a domain registrar and get a domain name. I2p sites have BASE32 names, kinda like onion addresses. But they can also use shorter names like postman.i2p
If you register such a name, the site will become more discoverable as those registrars likely share the sites, you might appear in some lists, people connect to you, add you to their address book and forward stuff to you.
If you want to host stuff, you probably want Termux. It is its own distribution but you may want to run a Debian proot inside Termux, which will have way more software and maybe also more reliably and fast security updates (heard that was a problem in Termux)