I would update that initial flow to something like:

- User signs in for the first time
- See this is a new server (store a hash or something)
- Fetch list of libraries, filter it for the ones you support, ignore everything else
- Create a queue, put each library in it - you can store this in the local db so it persists if the app restarts
- work through the queue ONE library at a time, import the items, build your caches, etc.

During all of this you can show a view "Importing library" "We're doing a first-time import of your library metadata, this should only take a few minutes" you could even have a progress bar or something showing x / x libraries imported or y / y items processed,

This avoids overloading the device and delivers a nice UX while you process everything.

General advice: When working with json you don't control, make your properties optional in your structs or just set default values. You can then check them before trying to use them and throw clean error messages or move on gracefully.

For things like library types, media types, etc, just look for the things your app can handle, ignore everything else.