I haven't used the iOS client, but on Android the most common way to sync is to use your own Git repository accessed over SSH. You could use a private GitHub repo or one on your own server.

same on iOS it authenticates to your git repo either with password or SSH key. The only "complexity" in setting the app up is giving your public and private GPG keys to the iOS app (you can transfer them using itunes - there's also a facility to scan a QR code, but I don't know how you generate that from your GPG credentials)

> there's also a facility to scan a QR code, but I don't know how you generate that from your GPG credentials

Search for qr on the python package index using pip. There's a module that you can pipe text to and then it'll render a qr code in your terminal using Unicode glyphs. Worked well last I tried. Don't remember what the module was called but you'll be able to find it I som sure.